Presence sync bugfix & Codable support for Broadcast and Presence API

[foodisbeast]

What kind of change does this PR introduce?

Bug fix, feature

What is the current behavior?

Presence .diff event is set to wrong ChannelEvent. This prevents registered onSync callbacks from executing.

public static let defaults = Options(events: [
  .state: .presenceState,
  .diff: .presenceState,
])

What is the new behavior?

Fixes .diff event replacing ChannelEvent.presenceState with ChannelEvent.presenceDiff

public static let defaults = Options(events: [
  .state: .presenceState,
  .diff: .presenceDiff,
])

Add alternative Encodable parameterization for Channel broadcast

public func broadcast(event: String, payload: Encodable, encoder: JSONEncoder = Defaults.encoder) throws -> Push

Add alternative Encodable parameterization for Channel Presence track

public func track(payload: Encodable, encoder: JSONEncoder = Defaults.encoder) throws -> Push

[foodisbeast]

Also added decode functions to Payload extension, and a new type BroadcastPayload. Let me know if you have any suggestions @GRSouza!

[grdsdev]

One minor improvement, but LGTM.

[foodisbeast]

Added new PushStatus enum

public enum PushStatus: RawRepresentable {
    case ok
    case error
    case timeout
    // ...
}