Document adding `claimId` to the execution engine JWT payload

[mjsmike62]

Document that claimId can be set in a JWT payload to convey information about the client being authenticated. This is useful for more advanced forms of authentication beyond the basic 1-CL-to-1-EL authentication that's usually done.

Also, document that adding claimId to the JWT payload for the execution engine is tested with ee-jwt-claim-id: foobar.

See PR: Add id claim to the execution engine JWT payload #7816

[StefanBratanov]

Hi, this has been done as part of #530