release-23.1.17-rc: release-23.1: CRDB-28040 : JWKS fetch from jwks_uri

[blathers-crl]

Backport 1/1 commits from #117054.

/cc @cockroachdb/release

---

Fixes CRDB-28040 ( JWKS fetch from jwks_uri )
This commit adds capability to fetch remote JWKS from issuer's jwks_uri endpoint. This will satisfy the requirement to have an ability to automatically fetch the new JWK when the existing JWK is rotated - without human intervention or custom scripts.

Changes include

1. The existing order of token signature verification first and rest of claims next is modified to get issuer first and then the token signature verification. This change is requied to determine the issuer for which the jwks has to be fetched remotely.

2. Introduction of a new cluster setting called server.jwt_authentication.jwks_auto_fetch.enabled

3. Depending on the value of server.jwt_authentication.jwks_auto_fetch.enabled use JWKS configured through cluster setting or remotely fetch JWKS from jwks_uri of the issuer

4. Modification to exiting test cases to match the new order of verification steps.

The change is backward compatible and not changes required in existing deployments and JWT Auth usage.

---

Release justification: high-priority need for functionality in 23.1

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is