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.