Add schema for changes in Group Type

Problem/Motivation

This module is saving some values in the third party settings of the Group Type Config Entity. We need to define this metadata in the schema configuration.

Proposed resolution

Add this schema configuration:

# Schema for the configuration files of the group_flex module.

group.type.*.third_party.group_flex:
  type: mapping
  label: 'Group flex settings'
  mapping:
    group_flex_enabler:
      type: 'boolean'
      label: 'Enable group flex'
    group_type_visibility:
      type: 'string'
      label: 'Group type visibility'
    group_type_joining_method:
      type: 'sequence'
      label: 'Group joining methods'
      sequence:
        type: 'string'
        label: 'The joining method'
    group_type_joining_method_override:
      type: 'boolean'
      label: 'Group owner can select the method of her/his "Group"?'

Make sure to remove the $strictConfigSchema property in the GroupFlexTest as well. There is no reason to ignore this in the tests anymore.