Expose `claims_supported` in oidc metadata document

### Proposal
I would like the nomad oidc discovery document to expose a `claims_supported` value.

### Use-cases
I am looking into integrating 1password in my nomad cluster through their new workload identity flow, but unfortunately they require the oidc server to expose the claims it supports instead of letting me type in claims to map manually. Exposing the claims that nomad usually emits in the JWT (`aud`, `nomad_namespace`, `nomad_job_id`, `nomad_allocation_id`, `nomad_service`, etc)

### Attempted Solutions
I have copied the openid configuration into a static file and modified it myself. Then I was able to route the request in my ingress load balancer to the manually maintained oidc metadata document. It works, but is clunky and prone to errors in case you update fields in the oidc metadata document in a new version.

Start by locating Nomad's OIDC discovery-document entry point and the tests covering its metadata response. Add a claims_supported value listing the claims Nomad emits in its JWTs, then verify the discovery document and its tests reflect the supported claims.

Written by the indexing model from the issue text.