Return the OIDC client ID as well as the issuer URL

[milliams]

Currently Conch will tell Clifton which URL it should use to authenticate (via /issuer), but Clifton still needs to be configured to know what client ID to use when talking to the issuer. Outside of some possible corner cases (which Clifton can always provide configuration for), there will only one client per issuer that will work and so Conch could just tell Clifton which one that is.

This will allow Clifton to only need a single piece of information about the CA (its base URL) and can retrieve everything else from there.

This will simplify the user configuration for isambard-sc/clifton#43.

The /issuer endpoint will need to remain as-is for backwards compatibility, but can be deprecated and removed in the future.