Improve signing key compatibility with other cryptography libraries

[zorgiepoo]

As discussed in #2467, our private signing key using https://github.com/orlp/ed25519 is not compatible with other libraries. This is because other libraries use seed or seed + public_key as their private key and instead oral/ed25519 hashes the seed so it doesn't have to re-hash it on every signing operation. Presently we discard the seed before the hashing operation so it is impossible to sign updates using other libraries which may be beneficial on other OS systems / toolchains / etc.

Proposal is that for newly generated keys, we preserve the seed. Potentially the new format could be seed + public_key and we would just derive the private key from the seed and public_key again. On export/import we may want to consider using a binary format instead of base64 but I'm not sure yet. Compatibility considerations may need to be made so older tools don't accidentally leak newly generated private keys.

[zorgiepoo]

We don't need to store the public_key. Changes to resolve this are in #2472