Allow `OpUp` custom fees

[cusma]

Problem

Currently the OpUp utility creates InnerTxns with minimum fee as default, consuming contract funds in case of insufficient pooled fees.

It would be useful to have an explicit option to make sure that InnerTxns fees for the budget top-up MUST be paid by the upper call.

Solution

I don't know if would be better to set it on OpUp instantiation like:

op_up = OpUp(mode=OpUpMode.OnCall, inner_fee=0)

rather than on OpUp usage like:

op_up.maximize_budget(fee=budget, inner_fee=0)

Dependencies

None.

Urgency

Not urgent.

[cusma]

@michaeldiamant just opened #553 with a proposal.