Enabling support for EMQX Shared Subscriptions.

[MichalNemec]

This pull request should enable EMQX Shared Subscriptions as per doc: https://docs.emqx.com/en/emqx/latest/messaging/mqtt-shared-subscription.html

Fixing #1808

[jeremydmiller]

Of the new CI tasks up above, we're only going to care about MQTT & the .NET task. CI "improvements" are not quite "Improved" yet

@MichalNemec Thank you!

[jeremydmiller]

@MichalNemec I'll pull this down locally after it's merged and do a sanity check before a release. Not sure the CI failures are relevant

[dmytro-pryvedeniuk]

Can it be that sample_listen_to_mqtt_topic_filter region was reused unintentionally in this PR? It results in an extra code snippet that does not correspond to the mentioned syntax.