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/source_metadata.go

@@ -0,0 +1,32 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package saml
+
+import (
+	"encoding/xml"
+	"fmt"
+	"net/http"
+)
+
+// Metadata redirects request/response pair to authenticate against the provider
+func (source *Source) Metadata(request *http.Request, response http.ResponseWriter) error {
+	samlRWMutex.RLock()
+	defer samlRWMutex.RUnlock()
+	if _, ok := providers[source.authSource.Name]; !ok {
+		return fmt.Errorf("provider does not exist")
+	}
+
+	metadata, err := providers[source.authSource.Name].samlSP.Metadata()
+	if err != nil {
+		return err
+	}
+	buf, err := xml.Marshal(metadata)
+	if err != nil {
+		return err
+	}
+
+	response.Header().Set("Content-Type", "application/samlmetadata+xml; charset=utf-8")
+	_, _ = response.Write(buf)
+	return nil
+}