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/index.js

@@ -23,7 +23,10 @@ import {initFindFileInRepo} from './features/repo-findfile.js';
 import {initCommentContent, initMarkupContent} from './markup/content.js';
 import {initPdfViewer} from './render/pdf.js';
 
-import {initUserAuthOauth2} from './features/user-auth.js';
+import {
+  initUserAuthOauth2,
+  initUserAuthSAML
+} from './features/user-auth.js';
 import {
   initRepoIssueDue,
   initRepoIssueReferenceRepositorySearch,
@@ -179,6 +182,7 @@ onDomReady(() => {
   initCaptcha();
 
   initUserAuthOauth2();
+  initUserAuthSAML();
   initUserAuthWebAuthn();
   initUserAuthWebAuthnRegister();
   initUserSettings();