Cater for the library usecase

Cater for the library usecase.

• Changed server start/await API to be more practical
• Revamped docs
• Added example unittest
• Improved Conf API ergonomy
• Added a dump_prefix config
• Switched mqttrs to upstream/crates.io release
• Updated tokio-util to 0.3

There is still plenty to do, but these changes were enough to convert my unittest to call the
library instead of running the binary.

First release

Mqttest is at this stage a niffty unittesting tool, even if some features are still missing. My
short-term plan is to work on the library usecase (as opposed to running a binary). Now is a great
time to chime in with your usecase, bug report, or code contribution.