Support subunits_per_unit returned by /v3/currency

[davidyuk]

Ideally, coin amounts returned by Node should be wrapped with an object with methods to convert it to different formats (fancy toString, coins, subunits). Also, it should be aware of subunits_per_unit to format properly.

To accept coin amounts, sdk should provide a class/utility function accepting amounts in units/subunits without knowing subunits_per_unit. The actual value of subunits_per_unit should be resolved internally using Node.

aeSdk.spend(new Coin(10), ...) // send 10 ae or coins with custom subunits_per_unit
aeSdk.spend(coin`10`, ...) // the same as above, alternative syntax
aeSdk.spend(10, ...) // send 10 aettos or custom subunits

But better to accept units by default instead subunits 🤔

related issue #1290