Custom Fields for Customer Groups

Customer groups can now be extended with custom fields. This allows users to categorize customer groups any way they want and filter the query result by custom fields.

• [API] Added a custom field to the CustomerGroup and CustomerGroupDraft.
• [API] Added Set Custom Type and Set Custom Field update actions to manage the custom fields on a customer group.
• [API] Added predicate for custom fields on Query CustomerGroups endpoint.