Signer

Signs a HashIDPreimageXdr and returns the resulting Signature.

Typical implementation:

1. Encode the preimage to bytes and hash with SHA-256

2. Sign the 32-byte hash with Ed25519

3. Return the public key and signature

The same preimage hash is used for every node in a WITH_DELEGATES tree; callers signing delegate nodes receive the same preimage as the top-level signer.