Allow user to choose spending strategy

[vforvalerio87]

Current behavior:
The sdk selects an open and funded channel and allows users to create channels; it does not deposit, create, fund or extend channels automatically for the user

Desired behavior:
Add a configuration option to allow the sdk to spend Ethereum (for transaction fees) and/or AGI (deposit, open channel, fund channel, extend channel) on behalf of the user, possibly within user-specified limits