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

---

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 0c56d32...4f80b26. Read the comment docs.