WIP: publish / subscribe / unsubscribe methods return a promise that resolves once message is sent (with respective QoS)

[FSMaxB]

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• [x] Formatted with cargo fmt
• [x] Make an entry to CHANGELOG.md if it's relevant to the users of the library. If it's not relevant mention why.

Explanation

This is the acked branch by @de-sh rebased onto main (as of now, it might fall behind over time).

The biggest issue with rebasing was a conflict with the FixedBitset change such that I had to remove some of them. I have some ideas on how a different API could change that though. I'll elaborate in #805.