[assets-controllers] Make `Token` type json-serializable

[MajorLift]

The Token type defined in the TokenRatesController has a balanceError property, which is typed as unknown. This property is assigned an error value in the TokensBalancesController updateBalances method.

TokensController uses the Token type in its state, which makes the state object non-json serializable. This is a roadblock for upgrading TokensController to BaseControllerV2 and needs to be fixed.