Add extension management

[vinistock]

Motivation

Closes #590

It will be good to allow clients to both display which Ruby LSP extensions are being used and also deactivate them if desired. Maybe an extension is failing and we want to disable it temporarily.

This PR adds a custom request that clients can use to discover which extensions have been registered and also starts accepting an initialization option with extension configuration, which clients can use to disable specific extensions.

Implementation

I split the implementation by commits.

1. Add a custom request that returns the discovered extensions
2. Starts accepting initialization options with extension configuration, so that we can skip activating disabled ones
3. Made an adjustment to our documentation mentioning that extensions should register their functionality all inside of activate. We'll need to update ruby-lsp-rails to match this, but I couldn't think of another way to de-register a listener that wouldn't involve messing up with the $LOAD_PATH and $LOADED_FEATURES

I'd appreciate feedback on the approach.

Automated Tests

Added some tests.

[github-actions]

Benchmark results in seconds (slowest at top)

          textDocument/completion average: 0.263018 std_dev: 0.005053
          textDocument/diagnostic average: 0.040978 std_dev: 0.008788
          textDocument/definition average: 0.004354 std_dev: 0.00293
      textDocument/selectionRange average: 0.003355 std_dev: 0.000489
   textDocument/documentHighlight average: 0.00183 std_dev: 0.0001
        textDocument/documentLink average: 0.001747 std_dev: 0.000106
            textDocument/codeLens average: 0.001736 std_dev: 0.000106
 textDocument/semanticTokens/full average: 0.001709 std_dev: 0.000263
      textDocument/documentSymbol average: 0.001703 std_dev: 7.3e-05
        textDocument/foldingRange average: 0.001598 std_dev: 0.000103
textDocument/semanticTokens/range average: 0.000858 std_dev: 7.4e-05
               codeAction/resolve average: 0.000649 std_dev: 5.1e-05
               textDocument/hover average: 0.000614 std_dev: 4.9e-05
           textDocument/inlayHint average: 0.000612 std_dev: 3.7e-05
          textDocument/formatting average: 6.9e-05 std_dev: 0.000285
    textDocument/onTypeFormatting average: 6.8e-05 std_dev: 4.0e-05
          textDocument/codeAction average: 4.0e-05 std_dev: 2.7e-05


================================================================================
Comparison with main branch:

 textDocument/semanticTokens/full unchanged
textDocument/semanticTokens/range unchanged
      textDocument/documentSymbol unchanged
        textDocument/foldingRange unchanged
          textDocument/formatting unchanged
          textDocument/diagnostic unchanged
        textDocument/documentLink unchanged
           textDocument/inlayHint unchanged
      textDocument/selectionRange unchanged
   textDocument/documentHighlight unchanged
               textDocument/hover unchanged
          textDocument/codeAction unchanged
    textDocument/onTypeFormatting unchanged
               codeAction/resolve unchanged
          textDocument/completion unchanged
            textDocument/codeLens unchanged
          textDocument/definition unchanged


================================================================================
Missing benchmarks:

RubyLsp::Requests::ShowSyntaxTree

[vinistock]

Putting this on pause from now until we have more use cases to learn from.