feat: Skin Changer

[JoshCantCode]

Description

Adds a Skin Changer for users to change their skin in-launcher

Documentation

This adds a skin changer page in the settings menu. Skins are saved by account, and can be activated, renamed, edited, you name it! Woahhhh. Very early progress, will update this description as I continue going. Just making a PR so people can shit talk whilst I make my limited rust knowledge destroy peoples brains

Does this PR require updates to the documentation at docs.polyfrost.org?
Nah

[LynithDev]

I'm not sure whether this is ready for review or not, could you request a review once you're done?

[JoshCantCode]

Frontend is still a mess. If its alright I might give you guys a ROUGH draft of the UI and then you can perfect it the way you want? I'm not familiar with freya at all

[JoshCantCode]

Here's what I hgave so far. Currently, the frontend is FUCKED up, bad.

I can't move the fetch button, and I want the preview to be inside of the dialog seperated by a divider. So you have the form and the buttons and text fields on the left, a vertical divider, and then the preview, but it won't work

Only other rthing (afaik) that needs to be added is the actual apply_skin functionality, I'll do that next