Describe the feature

Adding Custom Validators to Grants is doable by using the GrantType interface (see https://github.com/oauthlib/oauthlib/blob/v2.1.0/oauthlib/oauth2/rfc6749/grant_types/base.py#L49-L56). However, we provide pre_configured classes which does not expose "easily" these grant types. Also, we have multiple different kind of "hooks", for Server.__init__() we have token_generator=hook, token_expires_in=hook, refresh_token_generator=hook. Then for GrantTypeBase have custom_validators (dict) (pre_auth/post_auth/pre_token/post_token), register_code_modifier(hook) and register_token_modifier(hook).

It is complicated for OAuth2 provider implementation but become worst when using the OpenID Connect openid.pre_configured which has 8 GrantTypeBase and 3 Dispatchers, where each Dispatcher has two Grant with different names (e.g. default_auth_grant or default_implicit_grant).

E.g. Current OAuth2 Provider example:

oauthlib_server = oauth2.Server(..)
oauthlib_server._response_types["token"].custom_validators.pre_auth.append(myhook)
oauthlib_server._response_types["code"].custom_validators.pre_auth.append(myhook)
oauthlib_server._response_types["code"].register_code_modifier(mymodifier)

Same hooks with OpenID Provider is just impossible, or will span to more than 20 lines of code.

I open this issue to raise the problem and see what's the solutions we could imagine to progress toward a better place.

Additional context

It applies to OAuth2 Provider and OpenID Connect Provider server code.