Fix error when staking bond amount in chainspec is a bigint

[nathanwhit]

Currently if you have a chainspec with a staking bond amount that is large enough to be serialized as a bigint, you'll
hit an error in zombienet when calculating the balance for validator nodes:

Fail to add balance for nodes: [object Object],[object Object],[object Object],[object Object]

 Unexpected error:       TypeError: Cannot mix BigInt and other types, use explicit conversions

This is because the calculation is performed assuming the stakingBond is a number when it can, in fact, be a bigint in the chain spec.

This PR fixes this by performing the balance calculation entirely using bigints.

[pepoviola]

thanks @nathanwhit!