feat: add saml model & controller [CW-2958] (#12289)

This PR adds the foundation for account-level SAML SSO configuration in
Chatwoot Enterprise. It introduces a new `AccountSamlSettings` model and
management API that allows accounts to configure their own SAML identity
providers independently, this also includes the certificate generation
flow

The implementation includes a new controller
(`Api::V1::Accounts::SamlSettingsController`) that provides CRUD
operations for SAML configuration

The feature is properly gated behind the 'saml' feature flag and
includes administrator-only authorization via Pundit policies.

config/features.yml

@@ -204,3 +204,7 @@
   enabled: false
   premium: true
   chatwoot_internal: true
+- name: saml
+  display_name: SAML
+  enabled: false
+  premium: true