feat: explicit signatures

[erhant]

Currently we have two types of signed payloads:

• Task response is given with {encryption, signature} and the object itself is sent in plain
• Other signed messages are prepared as signature_bytes || message_bytes where parsing it is a bit more byte-manipulation instead of simple JSON-parse & read field.

We should unify all this and simply work with JSON formats.

[erhant]

We decide on {data, sig} JSON instead for all repsonses.

[erhant]

We will add signature and recovery_id to DriaMessage struct. Furthermore, the signed data will change from SHA256(decoded_payload) to SHA256(payload) in DriaMessage which simplifies the code & saves one extra decoding step.