feat: saml application configuration for login version (#9351)

# Which Problems Are Solved

OIDC applications can configure the used login version, which is
currently not possible for SAML applications.

# How the Problems Are Solved

Add the same functionality dependent on the feature-flag for SAML
applications.

# Additional Changes

None

# Additional Context

Closes #9267
Follow up issue for frontend changes #9354

---------

Co-authored-by: Livio Spring <livio.a@gmail.com>

internal/api/saml/serviceprovider.go

@@ -0,0 +1,53 @@
+package saml
+
+import (
+	"strings"
+
+	"github.com/zitadel/saml/pkg/provider/serviceprovider"
+
+	"github.com/zitadel/zitadel/internal/command"
+	"github.com/zitadel/zitadel/internal/query"
+)
+
+const (
+	LoginSamlRequestParam = "samlRequest"
+	LoginPath             = "/login"
+)
+
+type ServiceProvider struct {
+	SP                *query.SAMLServiceProvider
+	defaultLoginURL   string
+	defaultLoginURLV2 string
+}
+
+func ServiceProviderFromBusiness(spQuery *query.SAMLServiceProvider, defaultLoginURL, defaultLoginURLV2 string) (*serviceprovider.ServiceProvider, error) {
+	sp := &ServiceProvider{
+		SP:                spQuery,
+		defaultLoginURL:   defaultLoginURL,
+		defaultLoginURLV2: defaultLoginURLV2,
+	}
+
+	return serviceprovider.NewServiceProvider(
+		spQuery.AppID,
+		&serviceprovider.Config{Metadata: spQuery.Metadata},
+		sp.LoginURL,
+	)
+}
+
+func (s *ServiceProvider) LoginURL(id string) string {
+	// if the authRequest does not have the v2 prefix, it was created for login V1
+	if !strings.HasPrefix(id, command.IDPrefixV2) {
+		return s.defaultLoginURL + id
+	}
+	// any v2 login without a specific base uri will be sent to the configured login v2 UI
+	// this way we're also backwards compatible
+	if s.SP.LoginBaseURI == nil || s.SP.LoginBaseURI.String() == "" {
+		return s.defaultLoginURLV2 + id
+	}
+	// for clients with a specific URI (internal or external) we only need to add the auth request id
+	uri := s.SP.LoginBaseURI.JoinPath(LoginPath)
+	q := uri.Query()
+	q.Set(LoginSamlRequestParam, id)
+	uri.RawQuery = q.Encode()
+	return uri.String()
+}