Added support for custom handlers that can replace / work side-by-side with existing handlers

[david-driscoll]

Added support for class based handlers to HandlerTypeDescriptorHelper/LspHandlerTypeDescriptorHelper.
Added unit test demonstrating a custom params / handler pair for Execute Command
Made the OptionalAttribute's public
Added IExecuteCommandParams to allow for custom command execution handlers to work correctly.

fixes: #308

[codecov]

Codecov Report

Merging #309 into master will decrease coverage by 5.20%.
The diff coverage is 81.20%.

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   73.20%   67.99%   -5.21%     
==========================================
  Files         415      415              
  Lines       11743    11772      +29     
  Branches     1101     1105       +4     
==========================================
- Hits         8596     8004     -592     
- Misses       3147     3363     +216     
- Partials        0      405     +405     

Impacted Files	Coverage Δ
src/Server/Matchers/ExecuteCommandMatcher.cs	100.00% <ø> (ø)
src/JsonRpc/HandlerTypeDescriptor.cs	66.66% <28.57%> (-33.34%)	⬇️
src/Client/LanguageClientRegistrationManager.cs	50.96% <50.00%> (-5.77%)	⬇️
src/JsonRpc/HandlerTypeDescriptorHelper.cs	85.10% <81.81%> (-1.50%)	⬇️
.../Protocol/Shared/LspHandlerTypeDescriptorHelper.cs	86.36% <93.75%> (+2.15%)	⬆️
src/Dap.Shared/DebugAdapterHandlerCollection.cs	77.86% <100.00%> (-3.28%)	⬇️
src/JsonRpc/InputHandler.cs	84.63% <100.00%> (-5.45%)	⬇️
src/Shared/LspRequestRouter.cs	86.27% <100.00%> (-11.77%)	⬇️
src/Shared/SharedHandlerCollection.cs	72.42% <100.00%> (-3.28%)	⬇️
src/Protocol/Models/Hover.cs	50.00% <0.00%> (-50.00%)	⬇️
... and 162 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d53a6e...51b863d. Read the comment docs.