Add resource server support for multiple trusted JWT access token issuers

[dyroberts]

Summary

Add resource server support for multiple trusted JWT access token issuers

Actual Behavior

Presently we can configure a resource server's trusted jwt token issuer like:

security.oauth2.resource.jwt.keyUri=...
or
security.oauth2.resource.jwk.keySetUri=...

Expected Behavior

I'd like to suggest supporting something like (switching to yml):

security:
  oauth2:
    resource:
      issuers:
        issuer1:
          keyUri: ...
          issuerClaim: ...
        issuer2:
          keySetUri: ...
        issuer3:
          keyValue: ...

Note the optional issuerClaim for verification against the 'iss' claim, optionally.

[jgrandja]

The key question to ask the UAA team is the kid unique across identity zones?