Authorization Server Metadata and Dynamic Client Registration support

[zecakeh]

Hi, I wondered if there was any interest to add support for those two OAuth 2.0 RFCs:

1. Authorization Server Metadata (RFC8414): The main work would probably be to have a deserializable type for the metadata, since there is not mandatory endpoint to use to request it. It could even be used to build the Client, like Client::from_provider_metadata() in openidconnect. It would be very close to the ProviderMetadata from openidconnect, except that the required fields are different, like jwks_uri is not required for OAuth.
2. Dynamic Client Registration (RFC7591): this is one is a proper request with the registration_uri endpoint but the main work will probably be the type for the client metadata too. Here the ClientMetadata looks like it could fit.

Since openidconnect depends on this crate, maybe there can be a way to create the metadata types here and reuse them in openidconnect so they can be extended if necessary?