Allow for file assets customization instead of hardcoded icons

[fawni]

Is your feature request related to a problem? Please describe.
utils.get_asset_url is hardcoded to use icons from this repository. this makes the file_assets config option practically useless; as you cannot use any icons outside of this repository.

Describe the solution you'd like
Ideally, these icons should be uploaded to the (default) discord application instead of fetched from the github repository. this will also allow customization through custom discord applications as well as using external urls. this is the default behavior of presence.nvim.

[IogaMaster]

This is intended, as managing assets on discord sucks. When using urls updates are repo only (ie I don't need to do anything other than merging a pr).

To update the icons just override the file_assets. When it's fixed.
I will get to that soon