add proofs to queue message

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• Fetch merkle proofs (original and new leaves) and encode it in the queue message.
• A new file send_consumer.rs which consumes the queue and creates or loads a merkle tree which is meant to store the proven sends.
• Queue consumer from main.rs is gone.
• ECDSA verification is moved to a helper function and called from send API and on queue consumption.

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Generally looks good! Just want some answers before I LGTM.

Also, what's your general thinking about what checks should be done in model.rs vs send_consumer.rs? I'm seeing more "asserts" etc in model.rs than I expected, I thought that the send consumer would be doing most of the validation.

i am going with the idea that we only return promises for valid requests. If checks are done only on the consumer side, we'll return unfulfilled promises which we'll have to handle on the smart contract side making it gas expensive for valid broken promises. lmk if you have a way to circumvent this.

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@BlakeMScurr merging this as changing reverting the index changes has messed up the dependencies. you can still review this and i'll make any required changes in a coming pr.