[Currency] Add maximum amount to CurrencyPlugin

[Ishadijcks]

Currently there is no way to add a cap to currencies

Tasks

• Expand the CurrencyDefinition with a cap: number
• Create a test that asserts we cannot go over the cap

Future work

It seems reasonable to be able to add an upgrade that increases the cap, but I'm not sure if adding a modifier for it baked in is the right way to approach this....