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/forms/auth_form.go

@@ -1,3 +1,4 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
 // Copyright 2014 The Gogs Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
@@ -15,7 +16,7 @@ import (
 // AuthenticationForm form for authentication
 type AuthenticationForm struct {
 	ID                            int64
-	Type                          int    `binding:"Range(2,7)"`
+	Type                          int    `binding:"Range(2,9)"`
 	Name                          string `binding:"Required;MaxSize(30)"`
 	Host                          string
 	Port                          int
@@ -82,6 +83,18 @@ type AuthenticationForm struct {
 	SSPIDefaultLanguage           string
 	GroupTeamMap                  string `binding:"ValidGroupTeamMap"`
 	GroupTeamMapRemoval           bool
+
+	// SAML Settings
+	NameIDFormat                             int
+	IdentityProviderMetadata                 string
+	IdentityProviderMetadataURL              string
+	InsecureSkipAssertionSignatureValidation bool
+	ServiceProviderCertificate               string
+	ServiceProviderPrivateKey                string
+	EmailAssertionKey                        string
+	NameAssertionKey                         string
+	UsernameAssertionKey                     string
+	SAMLIconURL                              string
 }
 
 // Validate validates fields