`money` parameter in `startBuyerVerificationFlow` must be optional

[ggirotto]

Describe the issue

The money parameter in the buyer verification flow must be optional, because we may be verifying a saved card on file instead of a charge. I could see from the implementation that this property is not used if the action is Store, but either way it's non intuitive making it mandatory if it may not be.

[realmeylisdev]

Opened a fix for this in #281.

The money parameter is now optional (Money?) in both startBuyerVerificationFlow and startCardEntryFlowWithBuyerVerification. When buyerAction is "Store", you can simply omit it. A debug-mode assert ensures money is still provided when buyerAction is "Charge".