[feature] Add created_at and error_description to /oauth/token endpoint (#645)

* start fiddling about with oauth server

* start returning more helpful errors from oauth

* test helpful(ish) token errors

* add missing license header

internal/processing/oauth.go

@@ -0,0 +1,35 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package processing
+
+import (
+	"net/http"
+
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+func (p *processor) OAuthHandleAuthorizeRequest(w http.ResponseWriter, r *http.Request) error {
+	// todo: some kind of metrics stuff here
+	return p.oauthServer.HandleAuthorizeRequest(w, r)
+}
+
+func (p *processor) OAuthHandleTokenRequest(r *http.Request) (map[string]interface{}, gtserror.WithCode) {
+	// todo: some kind of metrics stuff here
+	return p.oauthServer.HandleTokenRequest(r)
+}