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>

routers/web/auth/openid.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/url"
 
+	auth_model "code.gitea.io/gitea/models/auth"
 	user_model "code.gitea.io/gitea/models/user"
 	"code.gitea.io/gitea/modules/auth/openid"
 	"code.gitea.io/gitea/modules/base"
@@ -363,7 +364,7 @@ func RegisterOpenIDPost(ctx *context.Context) {
 		Email:  form.Email,
 		Passwd: password,
 	}
-	if !createUserInContext(ctx, tplSignUpOID, form, u, nil, nil, false) {
+	if !createUserInContext(ctx, tplSignUpOID, form, u, nil, nil, false, auth_model.NoType) {
 		// error already handled
 		return
 	}
@@ -379,7 +380,7 @@ func RegisterOpenIDPost(ctx *context.Context) {
 		return
 	}
 
-	if !handleUserCreated(ctx, u, nil) {
+	if !handleUserCreated(ctx, u, nil, auth_model.NoType) {
 		// error already handled
 		return
 	}