(Backend) Fixing direct-send `POST /wallet/{walletname}/taker/direct-send` to accept a list of UTXOs instead of just the mixdepth

[amitx13]

Description: Subtask of: #689 and (JoinMarket-Org/joinmarket-clientserver#1561)

Aim: Allow users to have full control over their UTXOs when performing a direct-send.

Current Situation : Currently, the APIs POST: /wallet/{walletname}/taker/direct-send and POST: /wallet/{walletname}/taker/coinjoin use mixdepth to select unfrozen UTXOs and perform transactions. This means users cannot specify exact UTXOs, which limits granular selection. While freeze and unfreeze methods can be used to manage UTXOs, this does not provide the ideal coin control.

Objective : Modify the direct-send functionality to allow users to specify specific UTXOs, thus providing more precise control over their transactions . Once the API changes are merged, update the UI accordingly.