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

Fix #30266
Replace #31533

Author: lunny

Diff for: routers/init.go

@@ -47,6 +47,7 @@ import (
 	markup_service "code.gitea.io/gitea/services/markup"
 	repo_migrations "code.gitea.io/gitea/services/migrations"
 	mirror_service "code.gitea.io/gitea/services/mirror"
+	"code.gitea.io/gitea/services/oauth2_provider"
 	pull_service "code.gitea.io/gitea/services/pull"
 	release_service "code.gitea.io/gitea/services/release"
 	repo_service "code.gitea.io/gitea/services/repository"
@@ -144,7 +145,7 @@ func InitWebInstalled(ctx context.Context) {
 	log.Info("ORM engine initialization successful!")
 	mustInit(system.Init)
 	mustInitCtx(ctx, oauth2.Init)
-
+	mustInitCtx(ctx, oauth2_provider.Init)
 	mustInit(release_service.Init)
 
 	mustInitCtx(ctx, models.Init)