Add provider attribute to SAML group links

The GitLab API for adding a SAML group link supports an optional provider attribute to specify a unique provider name that must match for the group link to be applied. This property is currently missing from the AddGroupSAMLLinkOptions struct.

Reference: GitLab API documentation: https://docs.gitlab.com/api/saml/#add-a-saml-group-link

Proposed Solution:

• Add Provider *string to AddGroupSAMLLinkOptions.
• Add Provider string to the base SAMLGroupLink struct.
• Implement a unit test (TestAddGroupSAMLLinkWithProvider) to verify the behavior.