fix(#957): support iconhash/image data variant fields

[Mishura4]

WIP fix for #957
Do not squash

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[braindigitalis]

my only concern with this change is that it changes ALL iconhash, giving them ALL this new upload method, when only ONE icon in the entire of discord api supports this approach (guild banner). This seems to then portray to the user they can upload any icon this way (bot pfp, guild icon, etc) when they cant. There are also other api endpoints that support different methods of uploads (message creation, webhook creation - both these are massively different in the way they work with webhook being closest to what guild banner does here).
Do we have a way in place to prevent the upload functionality working in places where it should not?
Also is the variant smaller, or larger than the iconhash? I dont know the internals of variant, isnt it as big as its largest possible type in the template?

[braindigitalis]

Looking into the way theoretically a variant might work, the size of the icon type will be no larger than iconhash, because sizeof(iconhash) would be larger than sizeof(image_data). Therefore the only question remains is: how do we make it so that it doesnt seem to users like they could use the upload methods on a user pfp or something? Can we do something with a template and enable_if to directly flag uploadable fields and hide the methods elsewhere?

[Mishura4]

No this doesn't change all iconhash. Iconhash is unchanged.

• utility::iconhash: same as before. 16 bytes
• utility::image_data: ptr to std::byte[], size, dpp::image_type. 16 bytes (size is uint32_t to save space)
• utility::icon: wrapper around variant<icon_hash, image_data> with convenience methods (bool is_hash() etc). 24 bytes

Not sure yet if i need to replace all iconhash fields wirh icon. So far it's only the guild fields.
The variant does add some bytes for the index in the variant. Sadly no way around that unless we want to bet one part of the hash is never 0 or something like that (as discussed in Discord)
A bot can change their own pfp, this is something I'd like to support.

[Mishura4]

(please do not squash merge - many changes! 📈)

[braindigitalis]

When this is merged to dev, which examples in the dpp docs will need to be changed?

[Jaskowicz1]

When this is merged to dev, which examples in the dpp docs will need to be changed?

From how this PR looks, no docs need to be changed.