Allow payment method plugins to present themselves as multiple brands

Also make \Drupal\payment\Plugin\payment\method\Base provide a solution for site builders to configure and translate the human-readable names of any brands the method offers.

Patch is WIP. The problem is to make Base provide a user-friendly way that supports plugins with one and multiple brands, as the textfields will need to be grouped and need a short description, but putting one textfield in a details form element may be very invasive. Also, generic titles in 7.x-1.x (which was a primitive form of brand labels) defaulted to the specific title (now payment method entity label), which can now only be done if there is one brand, but then the behavior will be inconsistent between plugins with one brand, and those with more.

To do:
- Fix the aforementioned issue
- Upgrade specific titles for Basic payment methods to brand labels
- Update the predefined payment methods with brand labels

The default behavior, which is a single brand with the same name as the plugin itself.

When plugins override the defaults to define more than one brand. The default labels here are "Foo" and "Bar" and will always be displayed like that in the form element title. The values of these elements are how these brands are displayed to the payer, and these default to the default brand labels, but can be overridden.

Let's do this instead: no form elements by default, but Basic adds a textfield of its own to customize the brand label. The element is optional, and the brand label defaults to the payment method entity label.