Reduce use of Assembly.GetTypes() or ExportedTypes

[david-driscoll]

This adds a few generators that emit assembly attributes so that getting access to all the registration converters, and capability keys is working as expected.

[codecov]

Codecov Report

Merging #461 (b25380c) into master (d21f232) will decrease coverage by 0.36%.
The diff coverage is 50.14%.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   66.79%   66.42%   -0.37%     
==========================================
  Files         252      254       +2     
  Lines       11913    12186     +273     
  Branches      812      832      +20     
==========================================
+ Hits         7957     8095     +138     
- Misses       3956     4091     +135     

Impacted Files	Coverage Δ
.../JsonRpc.Generators/Cache/CachedSourceGenerator.cs	100.00% <ø> (ø)
src/JsonRpc.Generators/Helpers.cs	64.49% <ø> (+2.25%)	⬆️
....Generators/Strategies/HandlerGeneratorStrategy.cs	14.51% <ø> (ø)
src/JsonRpc/JsonRpcServerOptionsExtensions.cs	0.00% <0.00%> (ø)
src/Testing/ClientCapabilityExtensions.cs	6.25% <0.00%> (-0.90%)	⬇️
...erators/AssemblyCapabilityKeyAttributeGenerator.cs	4.10% <4.10%> (ø)
...ators/AssemblyJsonRpcHandlersAttributeGenerator.cs	5.12% <5.12%> (ø)
...DebugAdapterProtocolServiceCollectionExtensions.cs	82.14% <57.14%> (-8.77%)	⬇️
...sonRpc/JsonRpcServerServiceCollectionExtensions.cs	87.34% <57.14%> (-1.48%)	⬇️
src/Server/LanguageServer.cs	77.82% <83.33%> (-0.06%)	⬇️
... and 17 more

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