[clarity] UTXO Setters & Getters

[setzeus]

Description

The most important design change to the SIP involves the consoldiation during every deposit | withdrawal transaction to a single UTXO. This means that now the sBTC-wallet only ever holds a single UTXO at a time.

Therefore, the goal of this issue is to create a setter callable by any of the BTC-processor contracts (maybe we don't need all 3 anymore?) that updates the UTXO var. This issue should also include getters.

Acceptance Criteria

• Set consolidated UTXO
• Get consolidated UTXO