[WIP] [Contracts] Early payment proofs

[yeastplume]

Following on from experimental slate V5, changes, this aims to implement an experimental version of early proofs into the contracts branch. Note that `legacy' transaction workflows are not affected.

• Add required arguments structures and definitions
• Add initial test for development purposes
• (TBD)

Other notes

• signature for proof type Invoice is automatically generated by default. Generating another proof type or omitting proof requires opting out at present (subject to discussion)

[yeastplume]

This is mostly working now, note the new owner and foreign API functions to retrieve and verify proofs, as well as the contract_early_proofs test that demonstrates it all in action:

Note an exported proof with witness data (with no memo field) currently looks like this, a sender would provide this as well as the recipient wallet address for verification by any wallet's foreign API. A verifier looks up the given kernel, rebuilds the signature message from the given data and validates the signature.

{
  "proof_type": 1,
  "amount": "5000000000",
  "receiver_public_nonce": "02c6d5a13828310c1a3119b838e605f0c09894b8c293a3251e0256d5b281323d24",
  "receiver_public_excess": "03a325d02765d795b048b39fa0c4498ed8131fa331a620329b2c95de67e78733ff",
  "sender_address": "9eeead9b0e90c02e2db75b48e152252dab792b889f967329d695793605959a2d",
  "timestamp": 1685102955,
  "promise_signature": "9bd4030265fea7580fddb0d56fa115f1376186677b286dc855c067c68f5abc75cf701f29c36f027d0d9e2f0b51e00df37b92acf0aadf7ae012b2f30da8847d08",
  "witness_data": {
    "kernel_index": "16",
    "kernel_commitment": "086ff2f22ccfe3ea9b11d594dbf63e340d50c6d9841dca16437f0c9299a6322696",
    "sender_partial_sig": "4a80e076c5e89b1b689564439a56e0f9d906aae6d80bf79090fdcf11fb4311059879404578bc0a0927b677f5d773df308fdaec17590d3377e16e7c9744d5e9c1"
  }
}

[phyro]

I left a placeholder for proof generation in the setup phase: https://github.com/phyro/grin-wallet/blob/simple_contracts_restructured_v3/libwallet/src/contract/actions/setup.rs#L78. Similarly, the sign.rs has a placeholder to validate the proof https://github.com/phyro/grin-wallet/blob/simple_contracts_restructured_v3/libwallet/src/contract/actions/sign.rs#L81. The reason I structured them this way is because the receiver will always need to add the payment proof in the setup phase and the sender will have to verify it at the sign phase. This way it works for both invoice/payment flows. If we only add the proof at sign, it assumes the receiver signs before the sender.

[yeastplume]

sounds good and makes sense. I've moved the calls into the placeholder locations you had, going to add another test to make sure that both flows work!

[yeastplume]

Since PR is already large, going to merge this and start newer ones for cleanup work + additional testing