Implement MiddlewareSubscriber

[davidyuk]

related to #442

I had an idea to integrate https://www.npmjs.com/package/retry to automatically reconnect in case WS gets disconnected for whatever reason, but this would be unreliable since it is not possible to get emitted events since the connection is lost. So a reconnection should be implemented on aepp side.

Currently, I'm keeping middleware responses as they are. Would be better to reuse types from Node/swagger and numbers replace with bigints to don't lose precision aeternity/ae_mdw#1301
Ideally, middleware can provide encoded transactions and I would unpack them using transaction builder aeternity/ae_mdw#1187

Other than the current approach I've considered implementing it as:

class MiddlewareWebSocket extends WebSocket {

It was rejected because it is not possible to establish the connection on demand, WebSocket can't be reconnected.

Also, I've tried to use an existing EventEmitter class:

class MiddlewareSubscriber extends EventEmitter {
...
}

subscriber.on('transaction', <handler>);
subscriber.off('transaction', <handler>);
subscriber.on('microBlock:node', <handler>);

it was fine until l came to subscriber.on('ak_...:node', <handler>). There is no easy way in TS to append :node to ak_{string} having ak_{string}:node as the resulting type. Also doing this way the event emitter would ignore incorrect targets (technically, an arbitrary event name can be used), which obviously may appear if accepting the user input or so.

EventEmitter from Node can be replaced with EventTarget from browser js, but it has the same flaws and is more verbose.

This PR is supported by the Æternity Crypto Foundation

[davidyuk]

This package seems to be much more popular than websocket, it implements the same W3C api, I would migrate state channels to it later.
https://www.npmjs.com/package/ws
https://www.npmjs.com/package/websocket

[davidyuk]

_ means that this API is not stable and going to be changed.

[codecov]

Codecov Report

Patch coverage: 73.68% and project coverage change: -0.29 ⚠️

Comparison is base (3653b13) 82.68% compared to head (b51b0a3) 82.40%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1805      +/-   ##
===========================================
- Coverage    82.68%   82.40%   -0.29%     
===========================================
  Files           91       92       +1     
  Lines         3119     3194      +75     
  Branches       617      635      +18     
===========================================
+ Hits          2579     2632      +53     
- Misses         249      259      +10     
- Partials       291      303      +12     

Impacted Files	Coverage Δ
src/MiddlewareSubscriber.ts	72.97% <72.97%> (ø)
src/channel/Base.ts	76.47% <100.00%> (ø)
src/index-browser.ts	97.56% <100.00%> (-2.44%)	⬇️

... and 2 files with indirect coverage changes

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[thepiwo]

probably fine, didn't check in detail