split authentication out of jmap-core spec

[brong]

Need to make sure service discovery/endpoint discovery still works, as that is currently a response from the end of authentication.

[jeffpc]

Will signed GET requests get moved out as well?

[neilj]

Yes, these are part of authentication so they will be moved out.

[LoneFenris]

Apologies for the question here, but is the split spec for authentication available somewhere?

[neilj]

@LoneFenris We're not currently trying to standardise the authentication component, so we haven't published it anywhere sorry. The git history in this repo probably has the last version from when we had it as part of JMAP core though.