feat: banners can now be uploaded to bots

[Jaskowicz1]

This PR adds the ability to add banners to bots. I have also removed the limit imposed on avatars and banners as it seems the 256kb limit isn't true.

Code change checklist

• [x] I have ensured that all methods and functions are fully documented using doxygen style comments.
• [x] My code follows the coding style guide.
• [x] I tested that my change works before raising the PR.
• [x] I have ensured that I did not break any existing API calls.
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[Jaskowicz1]

forgor to cmake -B build lol

[Jaskowicz1]

Converted to draft, need to experiment with file limits as it seems avatars and banners do not have a 256kb constraint.

[Jaskowicz1]

No longer draft!

[Jaskowicz1]

I found that banners and avatars have no limit for bots? It could possibly be 25MB but Discord doesn't document this and it seems the request times out at anything over 9mb (at least it did for me). I assume it's a combination of can you upload in time and can Discord process it in time to respond, otherwise it throws a malformed HTTP request.

So, this PR now removes the limit for avatars and banners.