IHandlersManager and Handler linking improvments

[david-driscoll]

• Added extension method to get all the unique handlers contained in the IHandlersManager
• Renamed link parameter names to better reflect their meaning.
  • Added some better exceptions here to help users in the future.

[codecov]

Codecov Report

Merging #334 into master will increase coverage by 0.00%.
The diff coverage is 72.34%.

@@           Coverage Diff           @@
##           master     #334   +/-   ##
=======================================
  Coverage   75.21%   75.21%           
=======================================
  Files         420      421    +1     
  Lines       12249    12282   +33     
  Branches     1112     1118    +6     
=======================================
+ Hits         9213     9238   +25     
- Misses       3036     3044    +8     

Impacted Files	Coverage Δ
src/Client/LanguageClientOptions.cs	33.33% <0.00%> (ø)
src/Dap.Client/DebugAdapterClientOptions.cs	14.28% <0.00%> (ø)
src/Dap.Server/DebugAdapterServerOptions.cs	17.14% <0.00%> (ø)
src/JsonRpc/JsonRpcCommonMethodsBase.cs	47.16% <ø> (ø)
src/JsonRpc/JsonRpcOptionsRegistryBase.cs	80.00% <0.00%> (ø)
src/Server/LanguageServerOptions.cs	23.52% <0.00%> (ø)
src/JsonRpc/HandlerCollection.cs	68.83% <33.33%> (-7.29%)	⬇️
src/Dap.Shared/DebugAdapterHandlerCollection.cs	83.08% <100.00%> (+1.34%)	⬆️
src/JsonRpc/CompositeHandlersManager.cs	97.43% <100.00%> (ø)
src/JsonRpc/HandlersManagerExtensions.cs	100.00% <100.00%> (ø)
... and 4 more

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