[#1310] Add a configuration option for advertised LSP providers.

[the-mikedavis]

Description

Adds a configuration option providers with enabled and disabled lists. These can be used to configure which providers the language server should advertise to the client in the ServerCapabilities message. For example, to enable signature help:

# erlang_ls.config
providers:
  enabled:
    - signature-help

This could also be useful to bring in #980 as disabled-by-default.

The implementation is mostly inspired by the lenses configuration option which takes the same shape.

Fixes #1310.

[robertoaloi]

Thanks!