fix: Add OIDC clientAuthenticationMethod support for Druid

[dervoeti]

Description

Configures the druid.auth.pac4j.oidc.clientAuthenticationMethod property using the new ClientAuthenticationMethod enum from operator-rs, fixing the OIDC integration test that requires explicit client auth method (client_secret_post for Keycloak) and making it possible for users to specify this method in general.

Needs a new release of operator-rs that includes stackabletech/operator-rs@c81d453

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

Author

• Changes are OpenShift compatible
• CRD changes approved
• CRD documentation for all fields, following the style guide.
• Helm chart can be installed and deployed operator works
• Integration tests passed (for non trivial changes)
• Changes need to be "offline" compatible
• Links to generated (nightly) docs added
• Release note snippet added

Reviewer

• Code contains useful comments
• Code contains useful logging statements
• (Integration-)Test cases added
• Documentation added or updated. Follows the style guide.
• Changelog updated
• Cargo.toml only contains references to git tags (not specific commits or branches)

Acceptance

• Feature Tracker has been updated
• Proper release label has been added
• Links to generated (nightly) docs added
• Release note snippet added
• Add type/deprecation label & add to the deprecation schedule
• Add type/experimental label & add to the experimental features tracker