Add support for marshaling IObserver<T> over RPC

[AArnott]

Also:

• Fix proxies to accept multiple Dispose calls
• Centralize list of implicitly marshaled types across formatters

Closes #346

[codecov-commenter]

Codecov Report

Merging #536 into master will increase coverage by 0.24%.
The diff coverage is 97.72%.

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
+ Coverage   89.47%   89.72%   +0.24%     
==========================================
  Files          53       53              
  Lines        4258     4408     +150     
==========================================
+ Hits         3810     3955     +145     
- Misses        448      453       +5     

Impacted Files	Coverage Δ
src/StreamJsonRpc/JsonMessageFormatter.cs	91.17% <92.85%> (-0.11%)	⬇️
src/StreamJsonRpc/MessagePackFormatter.cs	91.69% <93.54%> (+0.02%)	⬆️
src/StreamJsonRpc/JsonRpc.cs	92.48% <100.00%> (-0.31%)	⬇️
src/StreamJsonRpc/JsonRpcProxyOptions.cs	90.00% <100.00%> (+1.11%)	⬆️
src/StreamJsonRpc/ProxyGeneration.cs	99.78% <100.00%> (+0.03%)	⬆️
...tion/MessageFormatterRpcMarshaledContextTracker.cs	96.19% <100.00%> (+3.76%)	⬆️
src/StreamJsonRpc/MessageHandlerBase.cs	100.00% <0.00%> (+1.21%)	⬆️

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