fix: add integrity protection for all messages

[djordon]

Description

Closes #517.

Changes

• Remove the digest field from the StacksTransactionSignRequest. It was always unnecessary.
• Sign messages through the protobuf encoding instead of through the wsts::net::Signable trait.
• Change SignEcdsa::sign_ecdsa to be infallible.
• Remove unnecessary Serialize/Deserialize implementations.

Testing Information

This is a refactor, so we should be good here.

Checklist:

• I have performed a self-review of my code

[aldur]

As instructed, marked as draft to avoid merging it early.

[matteojug]

Approving as we are relying on prost being deterministic enough. Opened #1001 as a followup to ensure we checked all the types.