Add objective C support to SwiftGen assets helpers

[aringenbach]

This adds Objective C annotations and converts SwiftGen assets objects from enum/structs to NSObject classes.
I made the update for most [UIImage imageNamed:] in Objective C classes.

[github-actions]

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[stefanceriu]

Looks great, well done! 👍

The only nitpicks I would comment on are that I would've used the dot notation everywhere and that there's a couple imageNamed: calls left in the RecentRoomTableViewCell and UnifiedSearchViewController

[stefanceriu]

Looks great! 👍

[aringenbach]

Made a few changes

• on the stencil (rename a param and add a missing access identifier)
• dot notation (my bad on that one, I kinda forgot that ObjC is sometimes convenient)
• actually just added the Images.swift to the RiotShareExtension target (the same way it's already done for NSE)

One way to improve from there would probably be to have a module/package dedicated to shared contents (in order to exclude properly other targets from snooping around Riot assets), but I guess that might be some work and not a huge impact.