Add Invoice and Subscription models for HubSpot billing support

[mapo-89]

This PR introduces two new models: Invoice and Subscription, to support HubSpot’s billing-related CRM objects.

✨ What’s included:

• STS\HubSpot\Crm\Invoice: Represents the HubSpot Invoice object
• STS\HubSpot\Crm\Subscription: Represents the HubSpot Subscription object
• Both follow the existing architecture used for models like Quote and LineItem

🔧 Implementation Notes:

• Located under src/Crm/
• Designed for consistency with the package’s existing API model structure
• Tested locally in standalone CLI usage against the live HubSpot API
• Sdk::$models array updated to include:
  • 'invoices' => Invoice::class
  • 'subscriptions' => Subscription::class

📚 References:

• HubSpot Developer Docs:
  • Invoices API
  • Subscriptions API

Thanks for reviewing! 🚀

[jszobody]

I assume you've tested these new models with your own HubSpot account? I don't use invoices/subscriptions with HubSpot, I don't have any data to test it with. But if you've interacted with the HubSpot API using these models, I'll go ahead and merge it in.

[mapo-89]

I’ve tested the invoices and subscriptions endpoints using my own production HubSpot account. The API integration is working reliably with no issues. From my perspective, everything is in order, so you can go ahead and merge it.