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 <46600784+stebenz@users.noreply.github.com>

internal/command/instance_idp_model.go

@@ -921,6 +921,7 @@ func (wm *InstanceSAMLIDPWriteModel) NewChangedEvent(
 	secretCrypto crypto.EncryptionAlgorithm,
 	binding string,
 	withSignedRequest bool,
+	signatureAlgorithm string,
 	nameIDFormat *domain.SAMLNameIDFormat,
 	transientMappingAttributeName string,
 	federatedLogoutEnabled bool,
@@ -934,6 +935,7 @@ func (wm *InstanceSAMLIDPWriteModel) NewChangedEvent(
 		secretCrypto,
 		binding,
 		withSignedRequest,
+		signatureAlgorithm,
 		nameIDFormat,
 		transientMappingAttributeName,
 		federatedLogoutEnabled,