feat: add saml request to link to sessions (#9001)

# Which Problems Are Solved

It is currently not possible to use SAML with the Session API.

# How the Problems Are Solved

Add SAML service, to get and resolve SAML requests.
Add SAML session and SAML request aggregate, which can be linked to the
Session to get back a SAMLResponse from the API directly.

# Additional Changes

Update of dependency zitadel/saml to provide all functionality for
handling of SAML requests and responses.

# Additional Context

Closes #6053

---------

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

internal/api/saml/auth_request.go

@@ -0,0 +1,99 @@
+package saml
+
+import (
+	"context"
+	"encoding/base64"
+	"net/url"
+
+	"github.com/zitadel/saml/pkg/provider"
+	"github.com/zitadel/saml/pkg/provider/models"
+	"github.com/zitadel/saml/pkg/provider/xml"
+
+	"github.com/zitadel/zitadel/internal/api/authz"
+	"github.com/zitadel/zitadel/internal/command"
+	"github.com/zitadel/zitadel/internal/domain"
+)
+
+func (p *Provider) CreateErrorResponse(authReq models.AuthRequestInt, reason domain.SAMLErrorReason, description string) (string, string, error) {
+	resp := &provider.Response{
+		ProtocolBinding: authReq.GetBindingType(),
+		RelayState:      authReq.GetRelayState(),
+		AcsUrl:          authReq.GetAccessConsumerServiceURL(),
+		RequestID:       authReq.GetAuthRequestID(),
+		Issuer:          authReq.GetDestination(),
+		Audience:        authReq.GetIssuer(),
+	}
+	return createResponse(p.AuthCallbackErrorResponse(resp, domain.SAMLErrorReasonToString(reason), description), authReq.GetBindingType(), authReq.GetAccessConsumerServiceURL(), resp.RelayState, resp.SigAlg, resp.Signature)
+}
+
+func (p *Provider) CreateResponse(ctx context.Context, authReq models.AuthRequestInt) (string, string, error) {
+	resp := &provider.Response{
+		ProtocolBinding: authReq.GetBindingType(),
+		RelayState:      authReq.GetRelayState(),
+		AcsUrl:          authReq.GetAccessConsumerServiceURL(),
+		RequestID:       authReq.GetAuthRequestID(),
+		Issuer:          authReq.GetDestination(),
+		Audience:        authReq.GetIssuer(),
+	}
+	samlResponse, err := p.AuthCallbackResponse(ctx, authReq, resp)
+	if err != nil {
+		return "", "", err
+	}
+
+	if err := p.command.CreateSAMLSessionFromSAMLRequest(
+		setContextUserSystem(ctx),
+		authReq.GetID(),
+		samlComplianceChecker(),
+		samlResponse.Id,
+		p.Expiration(),
+	); err != nil {
+		return "", "", err
+	}
+
+	return createResponse(samlResponse, authReq.GetBindingType(), authReq.GetAccessConsumerServiceURL(), resp.RelayState, resp.SigAlg, resp.Signature)
+}
+
+func createResponse(samlResponse interface{}, binding, acs, relayState, sigAlg, sig string) (string, string, error) {
+	respData, err := xml.Marshal(samlResponse)
+	if err != nil {
+		return "", "", err
+	}
+
+	switch binding {
+	case provider.PostBinding:
+		return acs, base64.StdEncoding.EncodeToString(respData), nil
+	case provider.RedirectBinding:
+		respData, err := xml.DeflateAndBase64(respData)
+		if err != nil {
+			return "", "", err
+		}
+		parsed, err := url.Parse(acs)
+		if err != nil {
+			return "", "", err
+		}
+		values := parsed.Query()
+		values.Add("SAMLResponse", string(respData))
+		values.Add("RelayState", relayState)
+		values.Add("SigAlg", sigAlg)
+		values.Add("Signature", sig)
+		parsed.RawQuery = values.Encode()
+		return parsed.String(), "", nil
+	}
+	return "", "", nil
+}
+
+func setContextUserSystem(ctx context.Context) context.Context {
+	data := authz.CtxData{
+		UserID: "SYSTEM",
+	}
+	return authz.SetCtxData(ctx, data)
+}
+
+func samlComplianceChecker() command.SAMLRequestComplianceChecker {
+	return func(_ context.Context, samlReq *command.SAMLRequestWriteModel) error {
+		if err := samlReq.CheckAuthenticated(); err != nil {
+			return err
+		}
+		return nil
+	}
+}