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/admin/auth/new.tmpl

@@ -53,6 +53,9 @@
 				<!-- SSPI -->
 				{{template "admin/auth/source/sspi" .}}
 
+				<!-- SAML -->
+				{{template "admin/auth/source/saml" .}}
+
 				<div class="ldap field">
 					<div class="ui checkbox">
 						<label><strong>{{ctx.Locale.Tr "admin.auths.attributes_in_bind"}}</strong></label>
@@ -85,6 +88,9 @@
 			<h5>GMail Settings:</h5>
 			<p>Host: smtp.gmail.com, Port: 587, Enable TLS Encryption: true</p>
 
+			<h5>SAML Settings:</h5>
+			<p>{{ctx.Locale.Tr "admin.auths.tips.saml"}}</p>
+
 			<h5 class="oauth2">{{ctx.Locale.Tr "admin.auths.tips.oauth2.general"}}:</h5>
 			<p class="oauth2">{{ctx.Locale.Tr "admin.auths.tips.oauth2.general.tip"}} <b id="oauth2-callback-url"></b></p>