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/web/web.go

@@ -667,6 +667,11 @@ func registerRoutes(m *web.Route) {
 			m.Get("/{provider}", auth.SignInOAuth)
 			m.Get("/{provider}/callback", auth.SignInOAuthCallback)
 		})
+		m.Group("/saml", func() {
+			m.Get("/{provider}", auth.SignInSAML) // redir to SAML IDP
+			m.Post("/{provider}/acs", auth.SignInSAMLCallback)
+			m.Get("/{provider}/metadata", auth.SAMLMetadata)
+		})
 	})
 	// ***** END: User *****