mcp: support url mode elicitation (#646)

Add initial support for URL mode elicitation, by introducing the new
types, as well as both server and client-side validation.

A follow-up CL will add support for ErrURLElicitationRequired.

For #623

mcp/shared.go

@@ -335,6 +335,9 @@ func clientSessionMethod[P Params, R Result](f func(*ClientSession, context.Cont
 const (
 	codeResourceNotFound = -32002
 	// The error code if the method exists and was called properly, but the peer does not support it.
+	//
+	// TODO(rfindley): this code is wrong, and we should fix it to be
+	// consistent with other SDKs.
 	codeUnsupportedMethod = -31001
 	// The error code for invalid parameters
 	codeInvalidParams = -32602