Validate aud payload

[hlab-pawat]

Summary

This PR adds support for validating the aud (audience) claim in JWT payloads, consistent with how exp and nbf claims are currently handled. To enable audience validation, both PayloadFields and the expected Aud value must be specified in the middleware configuration.

apiVersion: traefik.io/v1alpha1
kind: Middleware
metadata:
  name: jwt-auth
spec:
  plugin:
    jwt:
      PayloadFields:
        - aud
      Aud: my-api