Inform client of server extensions

[vinistock]

Once any gem can register an extension with the Ruby LSP, it may not be super clear what's going on in the server. We should start giving more visibility into this by informing the client about which extensions are running in the server.

Stretch goal for this issue: allowing turning an extension off if it's causing trouble.

I believe the easiest way of doing this would be sending a custom notification to the client (something like ruby_lsp/extensions) during extension activation. Then the client can handle that notification and display which extensions are available in the status centre.

[andyw8]

From talking with @vinistock:

We may also want to have a way to warn users if specific extensions are crashing (or causing performance problems).

[vinistock]

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

[vinistock]

This was already completed and the addons are now displayed in the language status item.