Move the firefox ios megazord into a subdirectory in ios-rust like focus

[tarikeshaq]

When creating the focus megazord, we put it in a subdirectory in megazords/ios-rust but kept the firefox-ios megazord in the root of ios-rust. It could be a good structural improvement to also put firefox-ios in a subdirectory

Originally posted by @skhamis in https://github.com/mozilla/application-services/pull/4953#discussion_r869772362

┆Issue is synchronized with this Jira Task