Support SAML authentication (#25165)

Closes https://github.com/go-gitea/gitea/issues/5512

This PR adds basic SAML support
- Adds SAML 2.0 as an auth source
- Adds SAML configuration documentation
- Adds integration test:
- Use bare-bones SAML IdP to test protocol flow and test account is
linked successfully (only runs on Postgres by default)
- Adds documentation for configuring and running SAML integration test
locally

Future PRs:
- Support group mapping
- Support auto-registration (account linking)

Co-Authored-By: @jackHay22

---------

Co-authored-by: jackHay22 <jack@allspice.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: morphelinho <morphelinho@users.noreply.github.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: silverwind <me@silverwind.io>

services/auth/source/saml/init.go

@@ -0,0 +1,29 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package saml
+
+import (
+	"context"
+	"sync"
+
+	"code.gitea.io/gitea/models/auth"
+	"code.gitea.io/gitea/modules/log"
+)
+
+var samlRWMutex = sync.RWMutex{}
+
+func Init(ctx context.Context) error {
+	loginSources, _ := auth.GetActiveAuthProviderSources(ctx, auth.SAML)
+	for _, source := range loginSources {
+		samlSource, ok := source.Cfg.(*Source)
+		if !ok {
+			continue
+		}
+		err := samlSource.RegisterSource()
+		if err != nil {
+			log.Error("Unable to register source: %s due to Error: %v.", source.Name, err)
+		}
+	}
+	return nil
+}