MQTT Transport

[didasy]

Adding MQTT transport that need shared subscription.

AFAIK shared subscription is supported by VerneMQ and EMQ.

[didasy]

Seems like NATS test failed from Travis:

=== RUN   TestDefaultTransport
=== RUN   TestDefaultTransport/testStandardTransportBehaviour
	test.go:128: 5 != 6
=== RUN   TestDefaultTransport/testSendChannelsDontBlock
--- FAIL: TestDefaultTransport (0.04s)
    --- FAIL: TestDefaultTransport/testStandardTransportBehaviour (0.03s)
    --- PASS: TestDefaultTransport/testSendChannelsDontBlock (0.01s)
=== RUN   TestStreamingTransport
=== RUN   TestStreamingTransport/testStandardTransportBehaviour
=== RUN   TestStreamingTransport/testSendChannelsDontBlock
--- PASS: TestStreamingTransport (0.14s)
    --- PASS: TestStreamingTransport/testStandardTransportBehaviour (0.14s)
    --- PASS: TestStreamingTransport/testSendChannelsDontBlock (0.00s)
=== RUN   TestReceive
--- PASS: TestReceive (0.02s)
=== RUN   TestSend
--- PASS: TestSend (0.01s)
FAIL
FAIL	github.com/matryer/vice/queues/nats	0.230s

[ojizero]

@JesusIslam hey dude 👋🏻 do you still want to add support to this transport? If so can you please update the branch to match V2 of Vice and check if NATS tests continue to fail on the new CI in GitHub Actions?