[stacks-signer] Retrieve pending deposits from the revealer API

[jferrant]

Description

Retrieve pending deposits from the revealer API and deserialize them into an appropriate struct

Context

In the happy path implementation of a signer, users do not have to submit a STX transaction and can simply submit a OP DROP tx through a Bridge. However, signers do not know of these transactions unless they query an API for them (this would also imply retrieving necessary info to "reveal" the commit transaction)

Depends on the Revealer API being defined (doesn't have to be implemented necessarily. Can mock the results)

Expected Approach

Query and parse the result returned from the revealer API

Acceptance Criteria

• Should have unit tests that mock the response of the revealer API for both success (empty list of txs or a list of txs) and failure cases