Channels: Make bridging more reactive

[danbarua]

The bridging api is a bit clunky at the moment:

await channel.BridgeTo(
  "user/1003", //destination
  bridgeOptions, //options
  (e) => ColorConsole.WriteLine("Bridge Progress Ringing...".DarkGreen()) // optionally notify on ringing
);

if (!channel.Bridge.IsBridged)
{
  // handle failure
}
else
{
  channel.Bridge.Channel.DoStuff();
}

In particular, we need a better way to handle externally initiated bridges, possibly via attended transfer, or maybe via a command from another EventSocket client.

For example, my use case is this:

When a channel is bridged to another channel,
I need to subscribe to Feature Codes on the b-leg (agent side) channel
So that I can pause/resume recording and initiate attended transfers

Give I have transferred the caller to another agent
When the caller is connected
Then the Feature Codes should be enabled on that agent's channel

The use of the word When in the above example implies that we can and should be handling this in a more functionally reactive manner. Instead of interrogating the Channel.Bridge status object, we can follow Tell Don't Ask by letting the Channel tell us when we have been bridged.

The proposed API could look like this:

channel.Bridge.Subscribe(async bridgedChannel =>
{
  bridgedChannel.DoStuff();
});

await channel.BridgeTo("destination");

So that's the happy path, nice and clean.

We need some way to notify of failure, and there are a few options for this.

1. Produce an exception in the Channel.Bridge observable

channel.Bridge.Subscribe(
  async bridgedChannel => { bridgedChannel.DoStuff(); },
  ex => { //handle ex.HangupCause etc }
);

2. Throw an exception in the Channel.BridgeTo Task:

csharp
try
{
await channel.BridgeTo("destination");
}
catch(BridgeFailedException ex)
{
//handle ex.HangupCause etc
}

I am not such a fan of (2) and (3) as currently, the only error handling that consumers of the library need to be aware of is `OperationCanceledException`, and the aim of this library is to allow people to write telephony applications with nice clean C# code. I do not want to be littering my code with ``try/catch`  any more than the bare minimum.

3) Expose `LastBridgeHangupCause` on the  Channel for interrogation, so the proposed api would look like this:

``` csharp
channel.Bridge.Subscribe(
  async bridgedChannel => { bridgedChannel.DoStuff(); }
);

await channel.BridgeTo("destination");
if (!channel.IsBridged)
{
   Console.WriteLine(channel.LastBridgeHangupCause);
  //otherwise success is handled in the Subscription callback
}

[danbarua]

Done.