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/externalaccount/link.go

@@ -7,9 +7,8 @@ import (
 	"context"
 	"fmt"
 
+	"code.gitea.io/gitea/models/auth"
 	user_model "code.gitea.io/gitea/models/user"
-
-	"github.com/markbates/goth"
 )
 
 // Store represents a thing that stores things
@@ -21,10 +20,12 @@ type Store interface {
 
 // LinkAccountFromStore links the provided user with a stored external user
 func LinkAccountFromStore(ctx context.Context, store Store, user *user_model.User) error {
-	gothUser := store.Get("linkAccountGothUser")
-	if gothUser == nil {
+	externalLinkUserInterface := store.Get("linkAccountUser")
+	if externalLinkUserInterface == nil {
 		return fmt.Errorf("not in LinkAccount session")
 	}
 
-	return LinkAccountToUser(ctx, user, gothUser.(goth.User))
+	externalLinkUser := externalLinkUserInterface.(auth.LinkAccountUser)
+
+	return LinkAccountToUser(ctx, user, externalLinkUser.GothUser, externalLinkUser.Type)
 }