[FLI-954] JWT Auth: Support custom claim validation

[markphelps]

Allow validating custom claims in JWT auth config

https://docs.flipt.io/configuration/authentication#claim-validation

We currently support validating iss and aud. We should probably add support for sub and secondly allow users to define custom claims that they want validated.

Perhaps under a custom: section?

authentication:
  methods:
    jwt:
      enabled: true
      validate_claims:
        issuer: https://auth0.com/
        audiences: https://flipt.io/, https://flipt.com/ 
        subject: foo
        custom:
           x: y
           a: b

Originally posted by @markphelps in https://github.com/flipt-io/flipt/issues/2945#issuecomment-2050687617

_FLI-954