Provide completion for `required_providers`

[radeksimko]

Current Version

0.12.0

Use-cases

With or without #370 the user would benefit from having an easy way to introduce a new provider into their configuration by just picking one from a completion box, instead of typing out or copy pasting the requirement entry such as

grafana = {
  source  = "grafana/grafana"
  version = "1.6.0"
}

Proposal

When completion is triggered

terraform {
  required_providers {
    <COMPLETION-HERE>
  }
}

we could list all preloaded providers there (later potentially all providers from the public Registry pulled via API) and add the entry on confirmation, e.g.

grafana = {
  source  = "grafana/grafana"
  version = "1.6.0"
}