How can an AS support both 2.0 and 2.1 clients concurrently

[aaronpk]

We may need some additional client registration metadata parameters in order to enforce some of the non-optional things in 2.1 while allowing 2.0 clients to still treat them as optional.

Related to #97

[aaronpk]

We need AS metadata to tell clients what kind of clients it supports: OAuth 2.1 clients, or both clients. If the parameter doesn't exist, clients can assume the server only supports 2.0.

AS metadata:

• oauth_versions: ["2.1","2.0"]

Should go in a new metadata section before extensibility https://www.ietf.org/archive/id/draft-ietf-oauth-v2-1-12.html#section-6

The client metadata needs to indicate whether the AS should follow 2.1 processing rules or 2.0 processing rules.

Client metadata

• oauth_version: "2.1"

Need a new client metadata section in https://www.ietf.org/archive/id/draft-ietf-oauth-v2-1-12.html#section-2.3.1