feat(saml): add SignatureMethod config for SAML IDP (#10520)

# Which Problems Are Solved
When a SAML IDP is created, the signing algorithm defaults to
`RSA-SHA1`.
This PR adds the functionality to configure the signing algorithm while
creating or updating a SAML IDP. When nothing is specified, `RSA-SHA1`
is the default.

Available options:
* RSA_SHA1
* RSA_SHA256
* RSA_SHA512


# How the Problems Are Solved

By introducing a new optional config to specify the Signing Algorithm. 

# Additional Changes
N/A

# Additional Context
- Closes #9842 

An existing bug in the UpdateSAMLProvider API will be fixed as a
followup in a different
[PR](https://github.com/zitadel/zitadel/pull/10557).

---------

Co-authored-by: Stefan Benz <[email protected]>

internal/command/instance_idp_test.go

@@ -5435,6 +5435,7 @@ func TestCommandSide_AddInstanceSAMLIDP(t *testing.T) {
 							[]byte("certificate"),
 							"",
 							false,
+							"",
 							nil,
 							"",
 							false,
@@ -5477,6 +5478,7 @@ func TestCommandSide_AddInstanceSAMLIDP(t *testing.T) {
 							[]byte("certificate"),
 							"binding",
 							true,
+							"http://www.w3.org/2001/04/xmldsig-more#rsa-sha256",
 							gu.Ptr(domain.SAMLNameIDFormatTransient),
 							"customAttribute",
 							true,
@@ -5500,6 +5502,7 @@ func TestCommandSide_AddInstanceSAMLIDP(t *testing.T) {
 					Metadata:                      validSAMLMetadata,
 					Binding:                       "binding",
 					WithSignedRequest:             true,
+					SignatureAlgorithm:            "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256",
 					NameIDFormat:                  gu.Ptr(domain.SAMLNameIDFormatTransient),
 					TransientMappingAttributeName: "customAttribute",
 					FederatedLogoutEnabled:        true,
@@ -5666,6 +5669,7 @@ func TestCommandSide_UpdateInstanceGenericSAMLIDP(t *testing.T) {
 								[]byte("certificate"),
 								"",
 								false,
+								"",
 								nil,
 								"",
 								false,
@@ -5705,6 +5709,7 @@ func TestCommandSide_UpdateInstanceGenericSAMLIDP(t *testing.T) {
 								[]byte("certificate"),
 								"binding",
 								false,
+								"",
 								gu.Ptr(domain.SAMLNameIDFormatUnspecified),
 								"",
 								false,
@@ -5850,6 +5855,7 @@ func TestCommandSide_RegenerateInstanceSAMLProviderCertificate(t *testing.T) {
 								[]byte("certificate"),
 								"binding",
 								false,
+								"",
 								gu.Ptr(domain.SAMLNameIDFormatUnspecified),
 								"",
 								false,