Adds configurable authentication converter for resource-servers with
token introspection (something very similar to what
JwtAuthenticationConverter does for resource-servers with JWT decoder).
The new (Reactive)OpaqueTokenAuthenticationConverter is given
responsibility for converting successful token introspection result
into an Authentication instance (which is currently done by a private
methods of OpaqueTokenAuthenticationProvider and
OpaqueTokenReactiveAuthenticationManager).
The default (Reactive)OpaqueTokenAuthenticationConverter, behave the
same as current private convert(OAuth2AuthenticatedPrincipal principal,
String token) methods: map authorities from scope attribute and build a
BearerTokenAuthentication.
Closes gh-11661
