MB-51664: JWT authentication

Add jwt_auth module to validate JWT.
This can be used to authenticate REST API requests.

Tested using jwt_tests cluster_tests (next commit).

Change-Id: I39620b1e132619265326b504ba0b09d1d0ac657b
Reviewed-on: https://review.couchbase.org/c/ns_server/+/219702
Tested-by: Neelima Premsankar <[email protected]>
Well-Formed: Build Bot <[email protected]>
Reviewed-by: Peter Searby <[email protected]>

Author: neelima32