[BUG] SamlAuthenticator expects exchange_key to be Base64 encoded

[DarshitChanpura]

What is the bug?

The new library switch from cxf to nimbus introduced a change in behavior of the format a key is accepted in. Prior to this change, the key didn't need to be base 64 encoded but with this change.

in 2.11 we don’t expect it to be encoded: https://github.com/opensearch-project/security/blob/2.11/src/main/java/com/amazon/dlic/auth/http/saml/AuthTokenProcessorHandler.java#L255
in main/2.x we expect it to be encoded: https://github.com/opensearch-project/security/blob/2.x/src/main/java/com/amazon/dlic/auth/http/saml/AuthTokenProcessorHandler.java#L255

This implies the library switch PR introduced a breaking change.

How can one reproduce the bug?
Steps to reproduce the behavior:
0. Start Opensearch and Opensearch Dashboards with security plugin installed.

1. Run saml_auth.test.ts without base64 encoded exchange key
2. You'll see an exception in opensearch logs: nested: IllegalArgumentException[Illegal base64 character 2d]; Full error logs here.

What is the expected behavior?
Should work without base-64 encoded exchange key.

Exit Criteria:

• Fix the breaking change
  OR
• Document the breaking change