Add "payment" dictionary for payment app specific properties

[sahel-sh]

This is a proposal for adding "payment" dictionary to web app manifest to contain payment app specific properties. Below are my thoughts:

1. There are some fields in the web app manifest that are currently used by payment apps only:
   Bring back "serviceworker" member #864 (comment)
2. Just-In-Time installable payment apps need to declare additional information as more features get added to the Payment Handler API. As an example supported_delegations field will allow JIT installable apps to declare the additional information that they can provide:
   https://github.com/sahel-sh/shipping-contact-delegation/blob/master/Explainer.md#edit-just-in-time-jit-installation
3. SInce there is an n:m relationship between payment apps and payment methods, adding app specific properties to payment method manifest does not seem right.

[dmengelt]

@marcoscaceres any chance to reconsider this proposal?