jwtauthccl: support JSON Pointer for identity claim lookup

[alex-berger]

Release note: Allow using values from nested JWT claims as user identity.

This change allows the identity claim setting to accept a JSON Pointer (RFC 6901) in addition to a simple claim name, enabling CockroachDB to extract identity from deeply nested token structures.

Motivation

Kubernetes ServiceAccount tokens store the serviceaccount UID inside a nested claim at /kubernetes.io/serviceaccount/uid rather than in a top-level field, see below example of a Kubernetes ServiceAccount Token (JWT). With this PR we can for example set server.jwt_authentication.claim to /kubernetes.io/serviceaccount/uid to use a ServiceAccount's unique identifier as user identity. So this is another mini-step towards #103110 - Scalable and flexible JWT based authentication.

{
  "aud": [
    "cockroachdb"
  ],
  "exp": 1772783261,
  "iat": 1772696861,
  "iss": "https://my-issuer/...",
  "jti": "d536bab9-f0fc-46ad-a09d-5d85ac85dce1",
  "kubernetes.io": {
    "namespace": "my-namespace",
    "node": {
      "name": "ip-10-176-16-155.eu-central-1.compute.internal",
      "uid": "6c9b214a-b6d8-43f9-b1e0-0e15b6126eda"
    },
    "pod": {
      "name": "my-workload-54b8897d8-88wzg",
      "uid": "c33ed74a-738c-499a-96d1-0bfdb68fee91"
    },
    "serviceaccount": {
      "name": "my-service-account",
      "uid": "136f6af7-4e5a-4891-b778-992ad1674b02"
    },
    "warnafter": 1772700468
  },
  "nbf": 1772696861,
  "sub": "system:serviceaccount:my-namespace:my-service-account"
}

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[cockroach-teamcity]

This change is