Implement optional authorization for token introspection endpoint

[NickUfer]

The RFC for the OAuth 2.0 Token Introspection states that the introspect endpoint MUST have some form authorization:

To prevent token scanning attacks, the endpoint MUST also require
   some form of authorization to access this endpoint, such as client
   authentication as described in OAuth 2.0 [RFC6749] or a separate
   OAuth 2.0 access token such as the bearer token described in OAuth
   2.0 Bearer Token Usage [RFC6750].

Some certified OAuth2 authorization servers have the introspect endpoint exposed on a different "internal" port (like ORY Hydra) which should be guarded by own measures. Some authorization servers use specific authorized clients for that.

Basic authorization should at least be supported.
Possible extra would be client credentials Bearer token authorization.