Read KMS Auth token from filepath

[anithapriyanatarajan]

Changes

Support for retrieving KMS provider Auth token from mount path configured with the new key signers.kms.auth.token-dir in configMap chains-config.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [x] Has Docs included if any changes are user facing
• [x] Has Tests included if any functionality added or changed
• [x] Follows the commit message standard
• [x] Meets the Tekton contributor standards (including functionality, content, code)
• [x] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [x] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Chains now supports extracting the KMS provider auth token (e.g., `VAULT_TOKEN`) from a specified filepath. 
Instead of using the configMap key `signers.kms.auth.token`, you can now use `signers.kms.auth.token-dir` pointing to a valid filepath within the container (e.g., `/etc/kms-secrets`).

### Steps to Utilize This Feature:
- Create a secret with the key name `KMS_AUTH_TOKEN`.
- Update the Chains deployment specification to volume mount this secret. 

**Note:** To project the latest token values without needing to recreate the pod, avoid using `subPath`.

[tekton-robot]

Hi @anithapriyanatarajan. Thanks for your PR.

I'm waiting for a tektoncd member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[PuneetPunamiya]

/ok-to-test

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	57.6%	6.4

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	57.6%	6.4

[PuneetPunamiya]

/test pull-tekton-chains-unit-tests

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3

[anithapriyanatarajan]

/test pull-tekton-chains-integration-tests

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3
pkg/chains/storage/docdb/docdb.go	76.0%	73.6%	-2.5

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3
pkg/chains/storage/docdb/docdb.go	76.0%	73.6%	-2.5

[anithapriyanatarajan]

/test pull-tekton-chains-unit-tests

[anithapriyanatarajan]

/test pull-tekton-chains-unit-tests

[anithapriyanatarajan]

/test pull-tekton-chains-unit-tests

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lcarva

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/chains/blob/main/OWNERS)~~ [lcarva] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[tekton-robot]

The following is the coverage report on the affected files. Say /test pull-tekton-chains-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/chains/signing/kms/kms.go	51.2%	55.6%	4.3
pkg/chains/storage/docdb/docdb.go	76.0%	73.6%	-2.5

[anithapriyanatarajan]

/test pull-tekton-chains-unit-tests

[anithapriyanatarajan]

/retest

[anithapriyanatarajan]

/test pull-tekton-chains-integration-tests

[lcarva]

/lgtm