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>

routers/init.go

@@ -35,6 +35,7 @@ import (
 	actions_service "code.gitea.io/gitea/services/actions"
 	"code.gitea.io/gitea/services/auth"
 	"code.gitea.io/gitea/services/auth/source/oauth2"
+	"code.gitea.io/gitea/services/auth/source/saml"
 	"code.gitea.io/gitea/services/automerge"
 	"code.gitea.io/gitea/services/cron"
 	feed_service "code.gitea.io/gitea/services/feed"
@@ -138,6 +139,7 @@ func InitWebInstalled(ctx context.Context) {
 	log.Info("ORM engine initialization successful!")
 	mustInit(system.Init)
 	mustInitCtx(ctx, oauth2.Init)
+	mustInitCtx(ctx, saml.Init)
 
 	mustInit(release_service.Init)