[stacks-signer] Retrieve challenge RevealInfo from the stacks node sbtc contract

[jferrant]

Description

Add a function to make a read only contract call to the .sbtc contract to retrieve the list of "pending deposits"

Context

These pending deposits are ones that were submitted to the node via a "challenge" stx transaction. This should include the BTC transaction id and the reveal info necessary to reveal it.

Expected Approach

Make a function to make the read only call to the appropriate contract Deserialize the result into a RevealInfo struct and add it to a todolist for the signer

Acceptance Criteria

• [ ] PR adds the feature ...
• [ ] Test the deserialization of the challenge struct and ensure it is correct