refactor(config): support hot reloading

[aeneasr]

This patch updates the config system to be replacable and uses functions instead of struct fields. This allows implementing hot reloading mechanisms easily.

BREAKING CHANGES: Please be aware that several internal APIs have changed, as well as public methods. Most notably, we added the context to all Write* metods.

 type OAuth2Provider interface {
-    WriteAuthorizeError(rw http.ResponseWriter, requester AuthorizeRequester, err error)
+    WriteAuthorizeError(ctx context.Context, rw http.ResponseWriter, requester AuthorizeRequester, err error)

-    WriteAuthorizeResponse(rw http.ResponseWriter, requester AuthorizeRequester, responder AuthorizeResponder)
+    WriteAuthorizeResponse(ctx context.Context, rw http.ResponseWriter, requester AuthorizeRequester, responder AuthorizeResponder)

-    WriteAccessError(rw http.ResponseWriter, requester AccessRequester, err error)
+    WriteAccessError(ctx context.Context, rw http.ResponseWriter, requester AccessRequester, err error)

-    WriteAccessResponse(rw http.ResponseWriter, requester AccessRequester, responder AccessResponder)
+    WriteAccessResponse(ctx context.Context, rw http.ResponseWriter, requester AccessRequester, responder AccessResponder)

-    WriteRevocationResponse(rw http.ResponseWriter, err error)
+    WriteRevocationResponse(ctx context.Context, rw http.ResponseWriter, err error)

-    WriteIntrospectionError(rw http.ResponseWriter, err error)
+    WriteIntrospectionError(ctx context.Context, rw http.ResponseWriter, err error)

-    WriteIntrospectionResponse(rw http.ResponseWriter, r IntrospectionResponder)
+    WriteIntrospectionResponse(ctx context.Context, rw http.ResponseWriter, r IntrospectionResponder)
 }

The default config struct has moved from package github.com/ory/fosite/compose.Config to github.com/ory/fosite.Config.

Please note that the HMAC / global secret has to be set no longer in the compose call, but in the config initialization:

-compose.ComposeAllEnabled(&compose.Config{}, store, secret, privateKey)
+compose.ComposeAllEnabled(&fosite.Config{GlobalSecret: secret}, store, privateKey)

Closes #666