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>

tests/integration/README.md

@@ -110,3 +110,20 @@ SLOW_FLUSH = 5S ; 5s is the default value
 ```bash
 GITEA_SLOW_TEST_TIME="10s" GITEA_SLOW_FLUSH_TIME="5s" make test-sqlite
 ```
+
+## Running SimpleSAML for testing SAML locally
+
+```shell
+docker run \
+-p 8080:8080 \
+-p 8443:8443 \
+-e SIMPLESAMLPHP_SP_ENTITY_ID=http://localhost:3003/user/saml/test-sp/metadata \
+-e SIMPLESAMLPHP_SP_ASSERTION_CONSUMER_SERVICE=http://localhost:3003/user/saml/test-sp/acs \
+-e SIMPLESAMLPHP_SP_SINGLE_LOGOUT_SERVICE=http://localhost:3003/user/saml/test-sp/acs \
+--add-host=localhost:192.168.65.2 \
+-d allspice/simple-saml
+```
+
+```shell
+TEST_SIMPLESAML_URL=localhost:8080 make test-sqlite#TestSAMLRegistration
+```