[C2F] Get user's liquidation address handler to also return the payment rail

[jakubcolony]

Specification

This is required by #2869.

To calculate crypto offramp transfer fees for other users (like payment recipients), we need to know their liquidation address's payment rail.

getUserLiquidationAddress handler should be modified to return the payment rail as part of its response.

[jakubcolony]

After clarifying with Arren, this might not be required for the initial implementation of #2869. Marking as on-hold.