feat: Custom transport mode

[magik6k]

This PR:

• Makes it possible to create a client with a callback allowing for use with custom transports
  • Curio needs this for doing RPC to a sub-process, I can see there being other neat uses for this too
• Adds a method on the Server handler which makes it possible to feed raw rpc frames into it and get responses, skipping the http layer
• Obviously adds a test
• Adds some docs to the README, also documenting reverse calling which I've noticed had zero docs

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.11111% with 14 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@81c1e3f). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #114   +/-   ##
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  Coverage          ?   71.88%           
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  Files             ?       11           
  Lines             ?     1494           
  Branches          ?        0           
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  Hits              ?     1074           
  Misses            ?      323           
  Partials          ?       97           

Files	Coverage Δ
server.go	76.38% <100.00%> (ø)
client.go	81.62% <58.82%> (ø)