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>

models/auth/oauth2.go

@@ -8,6 +8,7 @@ import (
 	"crypto/sha256"
 	"encoding/base32"
 	"encoding/base64"
+	"encoding/gob"
 	"fmt"
 	"net"
 	"net/url"
@@ -81,6 +82,10 @@ func Init(ctx context.Context) error {
 		builtinAllClientIDs = append(builtinAllClientIDs, clientID)
 	}
 
+	// This is needed in order to encode and store the struct in the goth/gothic session
+	// during the process of linking the external user.
+	gob.Register(LinkAccountUser{})
+
 	var registeredApps []*OAuth2Application
 	if err := db.GetEngine(ctx).In("client_id", builtinAllClientIDs).Find(&registeredApps); err != nil {
 		return err
@@ -605,21 +610,6 @@ func (err ErrOAuthApplicationNotFound) Unwrap() error {
 	return util.ErrNotExist
 }
 
-// GetActiveOAuth2SourceByName returns a OAuth2 AuthSource based on the given name
-func GetActiveOAuth2SourceByName(ctx context.Context, name string) (*Source, error) {
-	authSource := new(Source)
-	has, err := db.GetEngine(ctx).Where("name = ? and type = ? and is_active = ?", name, OAuth2, true).Get(authSource)
-	if err != nil {
-		return nil, err
-	}
-
-	if !has {
-		return nil, fmt.Errorf("oauth2 source not found, name: %q", name)
-	}
-
-	return authSource, nil
-}
-
 func DeleteOAuth2RelictsByUserID(ctx context.Context, userID int64) error {
 	deleteCond := builder.Select("id").From("oauth2_grant").Where(builder.Eq{"oauth2_grant.user_id": userID})