feat: added support for Premium App Subscriptions

[Jaskowicz1]

This PR adds all the new stuff for Premium App Subscriptions, allowing people to use discord's new app subscription stuff in their bots.

This PR is untested (hence the tickbox isn't ticked) and can't be tested due to no access to discord's monetisation (as I don't have a verified bot). We will only know this works when someone tests this with a verified bot (but that could be in months time).

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

[netlify]

✅ Deploy Preview for dpp-dev ready!

Name	Link
🔨 Latest commit	9f13f1f
🔍 Latest deploy log	https://app.netlify.com/sites/dpp-dev/deploys/6537a1c8fb011900085ae678
😎 Deploy Preview	https://deploy-preview-969--dpp-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[Jaskowicz1]

I can't properly test this PR due to the fact I do not have a verified bot, so i'm not applicable for the SKU/Entitlement stuff. Other than that, this PR is ready.