Support SEP-835

[pcarleton]

Recent spec change indicates how scopes should work:
modelcontextprotocol/modelcontextprotocol#835

Specifically, we'll want the SDK to support the client providing the scopes from a WWW-Authenticate header or from the PRM scopes_supported field.

[chipgpt]

@pcarleton I started a PR #983 to update for 4.1 Scope Selection Strategy. If this looks good, I can also work on 4.2 Enhanced Error Handling, do you know if this is needed?