Unknown encryption key provider

Detects non-existing encryption key providers references in OpenTofu encryption block configuration. E.g.for the code

terraform {
  encryption {
    key_provider "pbkdf2" "foo" {
      # Configuration parameters
    }
    method "aes_gcm" "yourname" {
      keys = key_provider.pbkdf2.unknown
    }
    method "aes_gcm" "yourname2" {
      keys = key_provider.pbkdf2.foo
    }
  }
}

we should check if the key provider reference is correct. In this case, unknown reference should be highlighted.

See https://opentofu.org/docs/language/state/encryption/#methods for more information.