[Currency] Add maximum amount to CurrencyPlugin

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....

Look for the CurrencyPlugin and CurrencyDefinition files in the codebase. The task involves adding a `cap: number` property to the CurrencyDefinition and writing a test to ensure the currency cannot exceed this cap. Start by examining existing tests for the CurrencyPlugin to understand the testing pattern.

Written by the indexing model from the issue text.