Support images w/ frontend globals

[grod220]

Unfortunately, the list of frontends are simply strings: https://github.com/prax-wallet/registry/blob/main/registry/globals.json#L40-L43. We should migrate to a new data structure that allows for images (similar to rpcs).

Given Prax has a live dependency on this data structure, we cannot simply change it. We'll need to create a new field, update the package, and move Prax to using that new field (after chrome store review). Maybe in a month or so, we can deprecate the old field.