fix: use internal tag for signature response type for backwards compatibility

[ghost]

The signature responses for ECDSA are non-backwards compatible after creating the enum type SignatureResponse for multiple signature schemes.
v1:

{
  "big_r": { "affine_point": "02C05BBFD4FB13618F96D4A38523C83773EC27EBD284E370708541D69AC125F21B" },
  "s": { "scalar": "6E4F121548CD389B21735B002DBBA1F0990BCDF3FCAB5A279D37AA6015083061" },
  "recovery_id": 0
}

v2

{
  "Secp256k1": {
    "big_r": { "affine_point": "03D707678DA8B6A9B6E70FD00E98810076FDAEAF1FF4C75BF475679B69FDCD97DC" },
    "s": { "scalar": "28620BFC193491ED21E94578FF7C6A17B431D59E22729E829748E427BDBF9CB8" },
    "recovery_id": 0
  }
}

To fix this we can encode the enum type internally, such that it becomes an additive change for old clients, preserving backwards compatibility:

{
  "scheme": "Secp256k1",
  "big_r": { "affine_point": "02C05BBFD4FB13618F96D4A38523C83773EC27EBD284E370708541D69AC125F21B" },
  "s": { "scalar": "6E4F121548CD389B21735B002DBBA1F0990BCDF3FCAB5A279D37AA6015083061" },
  "recovery_id": 0,
}