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>

templates/user/auth/signin_inner.tmpl

@@ -69,5 +69,22 @@
 		</div>
 	</div>
 	{{end}}
+	{{if .SAMLProviders}}
+	<div class="divider divider-text">
+		{{.locale.Tr "sign_in_or"}}
+	</div>
+	<div id="saml-login-navigator" class="gt-py-2">
+		<div class="gt-df gt-fc gt-jc">
+			<div id="saml-login-navigator-inner" class="gt-df gt-fc gt-fw gt-ac gt-gap-3">
+				{{range $provider := .SAMLProviders}}
+					<a class="{{$provider.Name}} ui button gt-df gt-ac gt-jc gt-py-3 saml-login-link" href="{{AppSubUrl}}/user/saml/{{$provider.Name}}">
+						{{.IconHTML 28}}
+						{{ctx.Locale.Tr "sign_in_with_provider" $provider.Name}}
+					</a>
+				{{end}}
+			</div>
+		</div>
+	</div>
+	{{end}}
 	</form>
 </div>