Add support for static bearer token authentication for KFP

[ptitzler]

This PR extends the existing Kubeflow authentication support. The new EXISTING_BEARER_TOKEN authentication type enables users to authenticate using a static token value. Elyra passes the specified token as-is to the server.

A Kubeflow Pipelines runtime configuration must be configured as follows to utilize this authentication type:

• select EXISTING_BEARER_TOKEN as Authentication Type
• specify a token value as Kubeflow Pipelines API Endpoint Password Or Token
• if a value for username is specified it is ignored

The token input field is masked by default.

Closes #3107

What changes were proposed in this pull request?

• Add support for new Kubeflow Pipelines authentication type
• Update runtime configuration topic in the user guide

How was this pull request tested?

• Reviewed output of make docs
• Added new authentication tests (elyra/tests/pipeline/kfp/test_kfp_authentication.py)
• Manual testing using RHOS deployment
  • Set up RH DSP test environment using instructions in https://github.com/HumairAK/odh-manifests/blob/odh-dsp-auth-demo/data-science-pipelines/base/auth/AUTH_NOTES.md
  • Created and submitted pipeline and confirmed that the API calls were successful