feat: multiarch promote

[yanksyoon]

Applicable spec: NA

Overview

• Adds a validator before calling the promote charm, which checks if the charmcraft.yaml contains the charm we're trying to promote
• Remove unused workflow get_runner_image.yaml

Rationale

• To allow multiarch build charm promotion.

Workflow Changes

• Adds new validate to callable promote_charm workflow

Checklist

• [x] The contributing guide was applied
• [x] The PR is tagged with appropriate label (urgent, trivial, complex)

The test cannot be done E2E because of the inputs. Hence, the script has been copied over and modified to be called with input arguments (rather than from github inputs.xxx). Modified script, try it out yourself! The test has been run against the following repository's charmcraft yaml

• GitHub runner's charmcraft.yaml (only charm with multi-arch support at the moment)
• Synapse
• Discourse-k8s
• Repo-policy-compliance

  
  yanks@nuc:~/canonical/github-runner-operator$ ./test.sh amd64 22.04 ubuntu
  yanks@nuc:~/canonical/github-runner-operator$ echo $?
  0

yanks@nuc:~/canonical/github-runner-operator$ ./test.sh arm64 22.04 ubuntu yanks@nuc:~/canonical/github-runner-operator$ echo $? 0

yanks@nuc:~/canonical/github-runner-operator$ ./test.sh amd64 24.04 ubuntu No matching base found: amd64 24.04 ubuntu

yanks@nuc:~/canonical/github-runner-operator$ ./test.sh amd32 22.04 ubuntu No matching base found: amd32 22.04 ubuntu

Backwards compatibility tests:
- Defined input `base-architecture`, `base-channel`, `base-name` to non existing values in charmcraft.yaml: https://github.com/canonical/jenkins-k8s-operator/actions/runs/10104355603/job/27943205874, fails as expected ✅ 
- Defined input `base-architecture`, `base-channel`, `base-name` to existing values in charmcraft.yaml: https://github.com/canonical/jenkins-k8s-operator/actions/runs/10104362266/job/27943222849, passes as expected ✅ 
- Undefined input `base-architecture`, `base-channel`, `base-name` to test backwards compatibility: https://github.com/canonical/jenkins-k8s-operator/actions/runs/10104383773/job/27943278395, passes as expected ✅ , chooses the first match by default.