refactor: use struct types for claim related types

BREAKING change.
The following types are changed from interface to struct type:

- AccessTokenClaims
- IDTokenClaims
- IntrospectionResponse
- UserInfo and related types.

The following methods of OPStorage now take a pointer to a struct type,
instead of an interface:

- SetUserinfoFromScopes
- SetUserinfoFromToken
- SetIntrospectionFromToken

The following functions are now generic, so that type-safe extension
of Claims is now possible:

- op.VerifyIDTokenHint
- op.VerifyAccessToken
- rp.VerifyTokens
- rp.VerifyIDToken

pkg/op/token_intospection.go

@@ -28,7 +28,7 @@ func introspectionHandler(introspector Introspector) func(http.ResponseWriter, *
 }
 
 func Introspect(w http.ResponseWriter, r *http.Request, introspector Introspector) {
-	response := oidc.NewIntrospectionResponse()
+	response := new(oidc.IntrospectionResponse)
 	token, clientID, err := ParseTokenIntrospectionRequest(r, introspector)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusUnauthorized)
@@ -44,7 +44,7 @@ func Introspect(w http.ResponseWriter, r *http.Request, introspector Introspecto
 		httphelper.MarshalJSON(w, response)
 		return
 	}
-	response.SetActive(true)
+	response.Active = true
 	httphelper.MarshalJSON(w, response)
 }