Field widget third party settings missing in schema

Problem/Motivation

When running functional tests and using config that uses "mask" third party settings, I get the following error:
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for core.entity_form_display.user.user.register with the following errors: core.entity_form_display.user.user.register:content.field_xxx.third_party_settings.mask missing schema

Steps to reproduce

1. Create a module that installs config that has mask third party settings on a field in the form display
2. Add a functional test that installs this module
3. Run the functional test

Proposed resolution

Add the missing widget settings in the schema.

Have a solution for this and will follow it with the code change shortly. Thanks for the great module!