Add support to properly handle custom extensions

[david-driscoll]

This finally unlocks the ability to contribute extensions with the library in a first class way.

• Added [CapabilityKey] to define the path on the Capabilities object that class is to derive from.
  • Existing Capabilities have been updated with the correct key
  • renamed the child classes of capabilities to make them distinct form the real Capability class
• ClientCapabilities, TextDocumentClientCapabilities, WorkspaceClientCapabilities, and WindowClientCapabilities have been updated to include ExtensionData fields containing any extra data that was provided by the json.
• Added a couple integration tests to ensure things are working expected.

[codecov]

Codecov Report

Merging #323 into master will increase coverage by 0.58%.
The diff coverage is 85.65%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   74.63%   75.21%   +0.58%     
==========================================
  Files         417      420       +3     
  Lines       12124    12251     +127     
  Branches     1105     1111       +6     
==========================================
+ Hits         9049     9215     +166     
+ Misses       3075     3036      -39     

Impacted Files	Coverage Δ
...Protocol/Client/Capabilities/ClientCapabilities.cs	100.00% <ø> (ø)
...Protocol/Server/Capabilities/ServerCapabilities.cs	100.00% <ø> (ø)
...rotocol/Shared/LspHandlerTypeDescriptorProvider.cs	93.10% <ø> (ø)
src/Protocol/Supports.cs	73.68% <ø> (-5.27%)	⬇️
...els/DocumentOnTypeFormattingRegistrationOptions.cs	37.50% <37.50%> (ø)
...rotocol/Models/SignatureHelpRegistrationOptions.cs	37.50% <37.50%> (ø)
src/Protocol/Models/RenameRegistrationOptions.cs	62.50% <62.50%> (ø)
src/Shared/SupportedCapabilities.cs	88.88% <66.66%> (-4.66%)	⬇️
.../Protocol/Models/DeclarationRegistrationOptions.cs	75.00% <75.00%> (ø)
...c/Protocol/Models/DefinitionRegistrationOptions.cs	75.00% <75.00%> (ø)
... and 67 more

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