proposal: golang.org/x/oauth2/resourcemeta: protected resource metadata

[jba]

Proposal Details

Protected Resource Metadata (https://datatracker.ietf.org/doc/html/rfc9728) is an OAuth2 extension enabling clients of resource servers to discover how to authenticate to those servers. It is a mandated part of the Model Context Protocol.

I propose adding it to the x/oauth2 repo. A preliminary implementation is at https://go.dev/cl/680156.

[jba]

/cc @rolandshoemaker