Swift Combine Publisher support

[johnnewman]

Description of the pull request

Hello! This pull request adds Combine Publisher support for Pusher events. This is done via an extension on the Pusher class that allows you to subscribe to channel events and global events. Below are some code examples of how this could be used.

Channel events:

let pusher = Pusher(key: "APP_KEY")
...
pusher
    .publisher(channelName: "my-channel", eventName: "my-event")
    .sink { event in
        // Do something with the PusherEvent. 
    }
    .store(in: &cancellables)

All global events:

pusher
    .publisher()
    .sink { event in
        // Do something with the global PusherEvent. 
    }
    .store(in: &cancellables)

Specific global events:

pusher
    .publisher(eventName: "my-event")
    .sink { event in
        // Do something with the global PusherEvent. 
    }
    .store(in: &cancellables)

Here's an example of a more advanced use case:

pusher
    .publisher(channelName: "my-channel", eventName: "my-event")
    .throttle(for: .milliseconds(500), scheduler: DispatchQueue.main, latest: true)
    .compactMap { $0.dataToJSONObject() as? [AnyHashable: Any] }
    .sink { myJSONObject in
        // do something
    }
    .store(in: &cancellables)

Why is the change necessary?

This provides an alternate approach to receiving events and is a convenient way of integrating Pusher into Combine streams.

[johnnewman]

👋 is there anything you need from me in order to merge this PR? Thank you!

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