Allow any profile type to be used as a "customer" profile type

Right now the "customer" profile type has some special behavior:
- The address field cannot be deleted.
- The address field's "Storage settings" cannot be changed.
- The address field's "Available countries" settings is hidden (since the value is taken from the store).
We are also missing another, as revealed in #3069941: after hide all the billing address field, there is a "Edit" button.....
- The address field should always be required (meaning that the setting should always be hidden)

In the future we want to allow different profile types to be used for billing VS shipping information, for sites which have that requirement.
This will be done in #3003525: Allow shipment types to specify a profile type. But for that to happen, we need to allow all of the above special behaviors to apply to any profile type.

So, we need to:
1) Introduce a "customer_profile_type" flag set via third party settings on the profile type.
2) Disable the flag checkbox on the profile type form for the built-in "customer" profile type.
3) When the flag is set, create the address field on the bundle. And vice-versa, delete the field when the flag is unset.
4) Modify all alters to check the flag instead of hardcoding the ID of the "customer" profile type.
5) Add an AddressBook::loadTypes() method which will return all customer profile types (by filtering via flag).