Investigate whether `ARTRestChannels` and `ARTRealtimeChannels` still needed in addition to `ARTChannels<ChannelType>`

[maratal]

Comment to ARTRestChannelsProtocol and ARTRealtimeChannelsProtocol states that:

// We copy this from the parent class and replace ChannelType by ARTRealtimeChannel * because // Swift ignores Objective-C generics and thinks this is returning an id, failing to compile. // Thus, we can't make ARTRealtimeChannels inherit from ARTChannels; we have to compose them instead.

It is from 2016, Swift has changed drastically since then.

┆Issue is synchronized with this Jira Task by Unito