Update MPC Contract Signature Arguments

[bh2smith]

The Near MPC Contract was recently updated (the whole interface has been modified) - specifically the arguments for the sign method: See here near/mpc#613

This contract is deployed to: https://testnet.nearblocks.io/address/v1.signer-prod.testnet

In this transaction: https://testnet.nearblocks.io/txns/E2fQfcdqPARCnok7RFAJ6WXur3SDKEB7ESgYiH5zHfBT

This PR

1. Adopts the new MPC contract sign method interface.
2. Lowers the default gas limit on the signature request (cf Lowered the minimum gas limit and upped the depth near/mpc#589)
3. Introduces a deposit of 1 YOCTO as is now required.
4. Updates the kdf (key derivation) to use sha3 instead of sha2 as outlined in https://github.com/near/mpc/blob/develop/CHANGELOG.md
5. Payloads are now all big endian instead of little endian (so no longer need to be reversed).

Note that the recovery ID is now returned with the MPC Signature, so we can remove A LOT of fluff related to signature "Recovery". There may still be more fluff that can be removed (or perhaps some may need to be reintroduced). Unfortunately the availability of the signing service is not stable enough to test everything. However, we have seen a successful request already: https://sepolia.etherscan.io/tx/0x1adfbaa3151ebdd69b790da41e719eaa039861bd7f56e34fd9f280bded63f535

This change is "breaking" in the sense that the MPC contract interface no longer accets the old Near Tx Payload. This means that the project will no longer work for users pointing at the v2.multichain-mpc.testnet as it does not appear to have been updated along with v1.signer-prod.testnet which we use by default here.

[DavidM-D]

This won't work with the V5 contract or the mainnet contract. Here's the full changelog for the v5 contract.
Broadly, don't reverse the hash scalar any more, don't reverse the key derivation scalar and use sha3 instead of sha2 for the key derivation. Happy to review, or we'll do it later this week on our libs.

[bh2smith]

Well that was a nightmare. BUT I GOT IT!