Moniroting: optional subaccount

[peterpeterparker]

The first version of the monitoring uses the main account to obtain cycles for ICP (if Mission Control cannot provide cycles). It would be beneficial to allow developers to use a subaccount. This way, they could have a main account for common wallet-related activities and a dedicated subaccount for cycles management.

Canfund will also support the cycles ledger, so we might even offer three options. However, I want to avoid making it overly complex.