OAuth 2.0 Protected Resource Metadata endpoint

[elena-shostak]

Summary

OAuth 2.0 Protected Resource Metadata endpoint https://datatracker.ietf.org/doc/rfc9728/

xpack.security.mcp.oauth2.metadata:
  authorization_servers:
    - <server1>
    - <server2>
  resource: <resource_uri>
  bearer_methods_supported:
    - header
    - body
    - query
  scopes_supported:
    - all
  resource_documentation: <resource_documentation_uri>

Checklist

• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines

Closes: https://github.com/elastic/kibana-team/issues/2750
Relates: #256182

[elasticmachine]

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