[SUBS] Add more extension points

[vmichalik]

Problem

• As a developer I'd like for my app to be able to easily respond to the widget through events and callbacks

Solution

• The solution should follow a clear use case as defined by a community project or target integration

[vmichalik]

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[tokdaniel]

I think this issue is underspecified.

Current callbacks:

• onButtonClick - fires when any button is pressed, the callback can filter to react to specific button presses based on the parameter buttonType which is passed in.
• onRouteChange - fires when we go from PaymentOption -> Wrap -> Review etc. Passed in parameter is the actual step we changed to.
• onTransactionSent -> fires when we sent a transaction, the passed in parameter is the transaction hash
• onSuccess -> fires when a stream has started
• onSuccessButtonClick - @deprecated in favor of onButtonClick
• onPaymentOptionUpdate - fires when the user changes paymentOptions

Lets specify what do we need.