feat: allow disabling of guild and channel caches

[braindigitalis]

This PR adds support for disabling the guild or channel cache completely, saving potentially gigabytes of ram at scale.
This needs some testing before it can be pushed to dev. I shall be testing it over the next few days.

Note that if you disable a cache, certain events may deliver a nullptr instead of the expected structure, e.g. if you disabled guild cache, on_guild_delete is going to give you nullptr as the guild being deleted as discord do not provide the 'old' data.

I have also added a new namespace dpp::cache_policy that contains three simple constexpr's as shorthand for generic cache policies that people might want to use: default, balanced, or none. Default is what DPP has always used, balanced is what triviabot is using right now and none is what it will use in the future.

users who are exclusively using slash commands and the resolved set there will be able to move to the disabled cache setting with minimal migration difficulty.

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	4dbd985
🔍 Latest deploy log	https://app.netlify.com/sites/dpp-dev/deploys/65144427a998d00008ab7446
😎 Deploy Preview	https://deploy-preview-895--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.

[Mishura4]

I don't really like the super abbreviated variable names and wish we'd use more explicit naming. Other than that I'm not familiar with the cache system in D++, but looks good to me generally.

[raxyte]

Can fix storing the role id vector in another PR. Thanks!