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

example/client/app/app.go

@@ -60,7 +60,7 @@ func main() {
 	http.Handle("/login", rp.AuthURLHandler(state, provider))
 
 	// for demonstration purposes the returned userinfo response is written as JSON object onto response
-	marshalUserinfo := func(w http.ResponseWriter, r *http.Request, tokens *oidc.Tokens, state string, rp rp.RelyingParty, info oidc.UserInfo) {
+	marshalUserinfo := func(w http.ResponseWriter, r *http.Request, tokens *oidc.Tokens, state string, rp rp.RelyingParty, info *oidc.UserInfo) {
 		data, err := json.Marshal(info)
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)