Add custom denom balance query

[neacsu]

This is needed so that we can query other denom balances, which is important especially for testing purposes at this moment.

[jstuczyn]

What do you think about doing it the other way around, i.e. have the get_balance take a Denom argument and create something like get_mixnet_balance that would call get_balance with the denom associated with the mixnet contract?

[neacsu]

What do you think about doing it the other way around, i.e. have the get_balance take a Denom argument and create something like get_mixnet_balance that would call get_balance with the denom associated with the mixnet contract?

Sounds good, I've pushed the patch