Add an external-signer path for third-party blocks (HSM/TPM/Secure Enclave support)

[echo-ilabs]

Closes #339.

create_block takes the signer's raw &PrivateKey, which rules out keys held in HSMs, TPMs and secure enclaves: they will sign bytes for you, but never release the key. This splits the operation in two, additively:

let unsigned = biscuit.third_party_request()?.prepare_block(block_builder)?;
let sig: Vec<u8> = hardware.sign(unsigned.bytes_to_sign());   // key never leaves the module
let block = unsigned.with_external_signature(public_key, &sig)?;
biscuit.append_third_party(public_key, block)?;

• prepare_block does everything create_block did up to the signature and exposes the exact bytes to sign.
• with_external_signature assembles the block, verifying the signature against the supplied public key up front -- so a wrong key or a corrupted signature fails right there instead of later, as an opaque deserialization error on the recipient's side.
• create_block is now built on top of the split path, so the two stay equivalent by construction.

No wire-format or verification changes. Tests cover external-signing round trips for Ed25519 and P-256 (the algorithm hardware modules typically hold, verified by full re-deserialization), byte-for-byte equivalence of the two paths for deterministic Ed25519, and tamper/wrong-key rejection at assembly.

[divarvel]

Hi, this is one of the goals of #334 which should be merged soon.

Can you have a look at it and check if that matches your use-case?