@type key() :: String.t()

@type signer() :: {type(), key()}

@type t() :: %Stellar.TxBuild.SignerKey{key: key(), type: type()}

@type type() :: :ed25519 | :sha256_hash | :pre_auth_tx | :ed25519_signed_payload

@type validation() :: {:ok, any()} | {:error, atom()}