Change chain extensions to return transferable balance

[prayagd]

Context

Related to this ticket. Fixes to be made so that Nabla modals should show up as transferrable balance.

Our chain extension exposes functions conforming to the ERC-20 interface. In ERC-20, the only function to get the balance of an address is using the balanceOf() call. At the moment, we are returning the 'free' balance of a user, see here. However, the free_balance is not necessarily the transferable balance, as it does not take into account the locked or reserved tokens.

See the relation of the system::account() query to the transferable amount.

TODO

Change the balance_of function of the chain extension to return the transferable amount ie. free - reserved - frozen.

[prayagd]

@ebma Please fill in the details

[ebma]

I updated the description @prayagd.