Forward initialization settings to addons

[vinistock]

Motivation

Closes #1204
First step to allow Shopify/ruby-lsp-rails#389

While we work super hard to minimize the amount of configuration for the Ruby LSP, it's not realistic to assume that addons are not going to need any settings to be exposed.

We need a way to allow addons to receive relevant editor settings, so that their behaviour can be customized.

Implementation

The idea is to allow for configuration objects like

"rubyLsp.addonSettings": {
  "Ruby LSP Rails": {
    "runtimeServerCommand": "bin/dev server",
    "somethingElse": false,
  }
}

And then we remember those settings in the global state. Addons will then have access to these settings and can decide what to do accordingly.

Automated Tests

Added tests.