Adapt for Ithaca + Tenderbake

Naive change based on these docs:

• https://docs.nomadic-labs.com/nomadic-labs-knowledge-center/how-to/prepare-your-baker-for-ithaca2#changes-regarding-the-bakers-signing-scheme

Example changes in other signers:

• tacoinfra/remote-signer: https://github.com/tacoinfra/remote-signer/commit/932186c5711e2f3d3b3694169b49c7eda781b284
• ecadlabs/signatory: https://github.com/ecadlabs/signatory/pull/154

Tests passing.