[stacks-signer] Retrieve pending withdrawals from the stacks node

[jferrant]

Description

Create a function to retrieve the list of pending withdrawal transactions to process

Context

Signers need to know what data they are compelled to process/what they need to do. Depends on function definition of the clarity contract call. These function calls probably require the caller to submit the reward cycle they are asking about

Expected Approach

Query the pox-4 contract using a readonly function call to retrieve and deserialize the expected info.

Acceptance Criteria

• [ ] Test that the function to call clarity read only deserializes the info correctly