Allow marshalable objects to implement additional optional interfaces

[matteo-prosperi]

No description provided.

[AArnott]

This is remarkable how well this fits in and how quickly you wrote it up. I have lots of comments, but I'm very enthusiastic about what you're proposing here.

[codecov-commenter]

Codecov Report

Merging #843 (e7c8bd7) into main (bd27af1) will increase coverage by 0.02%.
The diff coverage is 92.73%.

❗ Current head e7c8bd7 differs from pull request most recent head 8ed2b8d. Consider uploading reports for the commit 8ed2b8d to get more accurate results

@@            Coverage Diff             @@
##             main     #843      +/-   ##
==========================================
+ Coverage   91.67%   91.69%   +0.02%     
==========================================
  Files         134      135       +1     
  Lines       13343    13689     +346     
==========================================
+ Hits        12232    12552     +320     
- Misses       1111     1137      +26     

Flag	Coverage Δ
LinuxHost	90.73% <92.73%> (+0.03%)	⬆️
Release	91.69% <92.73%> (+0.02%)	⬆️
WindowsHost	91.67% <92.73%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/StreamJsonRpc/Reflection/RpcTargetInfo.cs	95.03% <86.95%> (+0.81%)	⬆️
test/StreamJsonRpc.Tests/MarshalableProxyTests.cs	91.22% <88.79%> (-2.02%)	⬇️
...tion/MessageFormatterRpcMarshaledContextTracker.cs	96.37% <95.38%> (-0.73%)	⬇️
src/StreamJsonRpc/ProxyGeneration.cs	99.41% <98.51%> (-0.38%)	⬇️
src/StreamJsonRpc/JsonMessageFormatter.cs	91.51% <100.00%> (-0.02%)	⬇️
src/StreamJsonRpc/JsonRpc.cs	92.50% <100.00%> (+0.44%)	⬆️
src/StreamJsonRpc/MessagePackFormatter.cs	92.45% <100.00%> (-0.01%)	⬇️
...ection/RpcMarshalableOptionalInterfaceAttribute.cs	100.00% <100.00%> (ø)
src/StreamJsonRpc/HeaderDelimitedMessageHandler.cs	86.98% <0.00%> (-0.60%)	⬇️
... and 4 more

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[AArnott]

Exciting stuff. I still need to review the ProxyGeneration and tests, but thought I'd offer my feedback so far.
Note I did push a commit to your branch with a few small changes I just made instead of commenting asking you to make them.

[AArnott]

Finished reviewing.