Add support to disable settings through `didChangeConfiguration`

[aspeddro]

Some settings can be disabled by the user. Currently the server has no means of disabling these features at the server level.

VSCode can disable codelens through the following setting:

"[ocaml]": {
  "editor.codeLens": false
}

However other clients may not have means of disabling settings, such as Neovim.

Suggested settings for the user to control

• Codelens
• Hover Extended

Example config schema

"ocaml.codelens.enable": false,
"ocaml.hover.extended.enable": true

[rgrinberg]

Sure, that's a reasonable request. Feel free to send a PR.