[Cash App] Add support for adding Cash App tokens via My Account Payment methods

[james-allan]

Describe the solution you'd like

After #3156 was merged, Cash App can now be used to purchase Subscriptions, and Cash App payment methods can be saved via the checkout.

Given Cash App Pay can be saved via the checkout, we should add support for adding tokens via the My Account → Payment methods.

There are currently 2 bugs in this:

1. Cash App isn't displayed on the My Account → Payment methods page unless you have a product in the cart.
2. Attempting to add a Cash App token results in an error.

Both errors should be fixed and general support for adding Cash App tokens via this page should be added.