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>

web_src/js/features/user-auth.js

@@ -20,3 +20,24 @@ export function initUserAuthOauth2() {
     });
   }
 }
+
+export function initUserAuthSAML() {
+  const outer = document.getElementById('saml-login-navigator');
+  if (!outer) return;
+  const inner = document.getElementById('saml-login-navigator-inner');
+
+  checkAppUrl();
+
+  for (const link of outer.querySelectorAll('.saml-login-link')) {
+    link.addEventListener('click', () => {
+      inner.classList.add('gt-invisible');
+      outer.classList.add('is-loading');
+      setTimeout(() => {
+        // recover previous content to let user try again
+        // usually redirection will be performed before this action
+        outer.classList.remove('is-loading');
+        inner.classList.remove('gt-invisible');
+      }, 5000);
+    });
+  }
+}