Include payment handler information in response

[ianbjacobs]

We have heard use cases for merchants having information about the user's selected payment handler for a transaction, including:

• Being able to debug issues
• Security / Trust in some payment handlers

This is most useful for standardized payment methods but may also be useful for URI-identified payment methods that allow for multiple payment handlers.

The original payment handler issue is 217: https://github.com/w3c/payment-handler/issues/217

I am not proposing this for a v1 feature. However, when we start to consider the issue we can return to a proposal to return two bits of information in the PR API response:

handlerType: {'built-in', 'web', 'native') handlerID: {null, <origin>, <platform-specific> }

Ian