Make oauth2 code clear. Move oauth2 provider code to their own packages/files (#32148)

Fix #30266
Replace #31533

services/oauth2_provider/init.go

@@ -0,0 +1,19 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package oauth2_provider //nolint
+
+import (
+	"context"
+
+	"code.gitea.io/gitea/modules/setting"
+)
+
+// Init initializes the oauth source
+func Init(ctx context.Context) error {
+	if !setting.OAuth2.Enabled {
+		return nil
+	}
+
+	return InitSigningKey()
+}