ATO-2559: Add feature flag for defaulting tokenAuthMethod

[cearl1]

Wider context of change

We previously allowed for tokenAuthMethod to be null, and treated private_key_jwt as the default in the PrivateKeyJwtClientAuthValidator. To make it a bit clearer it would be nice to move this defaulting into the client registry getter.

What’s changed

This PR adds a feature flag (enabled in dev, build, and staging) to default the tokenAuthMethod to private_key_jwt if it is null. This is done in a utility class and the method is used in all places where we access the tokenAuthMethod. I also added some tests for the utility class.

Checklist

• Lambdas have correct permissions for the resources they're accessing.
• Impact on orch and auth mutual dependencies has been checked.
• Changes have been made to contract tests or not required.
• Changes have been made to the simulator or not required.
• Changes have been made to stubs or not required.
• Successfully deployed to authdev or not required.
• Successfully run Authentication acceptance tests against sandpit or not required.
• Added new endpoints to local running (LocalOrchestrationApi.java) or not required.

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
97.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud