Allow fetching by texture IDs

[violet-sh]

Closes #25. This allows you to fetch any resource by its texture ID.

There are two main things to consider with this pr:

1. Using solely texture IDs, there is no way of knowing whether the skin is of the "slim" model variety or not. This is for the most part a non-issue, however, it causes problems with body rendering. It also means that any request by texture ID will return anX-Crafthead-Skin-Model header of default.
2. There is also no way of knowing whether the texture is for a skin or cape. Currently, the pr just yolo's it, and requesting a cape just renders the texture as a cape even if it is a skin, and vice versa. This can cause very cursed images, but I guess if someone's requesting texture IDs directly they've probably done the due diligence of knowing what they need.

[violet-sh]

Given people have to know what texture ID to fetch anyway, it's not unreasonable to expect them to know whether or not a skin needs to have a slim model. If we can assume that, it might be helpful to provide a way to request a slim model, perhaps with a query parameter such as ?slim or ?model=slim.

[FN-FAL113]

This would be handy and opens up support for custom skins that are cached in the texture.minecraft.net/

[FN-FAL113]

bump

[Hazmi35]

Hello. What are the status on this PR?

[Cherry]

Apologies for the delay in getting back to this. We've done a lot of maintenance on this repo since this PR was opened. Is this something folks are still wanting/needing?