Change implicit StreamObserver conversions to syntax

[peterneyens]

Small internal cleanup

• Conversions between StreamObserver and Monix / reactive streams Subscribers are now syntax methods instead of implicit conversions.
• Introduce transformStreamObserver.

[juanpedromoreno]

Great improvements, thanks!

[codecov-io]

Codecov Report

Merging #157 into master will decrease coverage by 4.72%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   92.65%   87.93%   -4.73%     
==========================================
  Files          30       30              
  Lines         286      290       +4     
  Branches        6        7       +1     
==========================================
- Hits          265      255      -10     
- Misses         21       35      +14

Impacted Files	Coverage Δ
...es/internal/src/main/scala/server/monixCalls.scala	80% <100%> (-4.22%)	⬇️
...odules/internal/src/main/scala/MonixAdapters.scala	80% <100%> (ø)	⬆️
...dules/internal/src/main/scala/server/package.scala	77.77% <100%> (+2.77%)	⬆️
...ules/internal/src/main/scala/server/fs2Calls.scala	85.71% <100%> (-7.15%)	⬇️
...dules/internal/src/main/scala/client/package.scala	100% <100%> (ø)	⬆️

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