`ShowMessageRequestClientCapabilities` should implement `ICapability`

[chrissimon-au]

When attempting to use the LanguageServerTestBase to construct a LanguageClient for e2e testing of my language server, I'd like to test different scenarios for the client supporting message requests and not.

Unfortunately, ShowMessageRequestClientCapabilities doesn't implement ICapability, so when trying to add it to the client options with WithCapability it fails due to the type requirement.

This seems like a simple fix, so I'll be linking a PR shortly.