Add a simplistic SparkplugBSession implementation

[nightscape]

It mostly just forwards to the standard MQTT sessions and converts the messages to and from the Sparkplug B protocol.

[nightscape]

@fcabestre I manually tested this locally with a JS Sparkplug B client and it worked nicely. Unfortunately I haven't found the time to write automated tests yet. I need to create a PoC using this first, then I should be able to invest more time to make it nicer and add tests. Would it be ok from your side to merge this as is, and I make additional PRs whenever I find time to work on it?

[codecov-commenter]

Codecov Report

Merging #91 (946bd6b) into master (f61715f) will decrease coverage by 6.52%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
- Coverage   29.55%   23.02%   -6.53%     
==========================================
  Files          17       19       +2     
  Lines         247      317      +70     
  Branches        6        6              
==========================================
  Hits           73       73              
- Misses        174      244      +70     

Impacted Files	Coverage Δ
.../scala-2/net/sigusr/mqtt/sparkplug/ToMetrics.scala	0.00% <0.00%> (ø)
.../net/sigusr/mqtt/sparkplug/SparkplugBSession.scala	0.00% <0.00%> (ø)

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[fcabestre]

Well, contributions are always welcome and I'll happy to merge this early work. Just spare me some time to wrap my head around this and see how to proceed.

[nightscape]

Of course! I'm making some enhancements to the ToMetrics typeclass, I'll let you know once it's mergeable from my side.