Add a new `template_category_update` webhook subscription field

[Bindambc]

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Add implementation for the features described in the API changelog (March 17, 2023):

Applies to all Business Management API versions.

• Added a new template_category_update webhook subscription field. If subscribed to this field, anytime a template's category changes you will receive a webhook indicating the template's previous and new category.
• API responses to template creation and editing requests now include the template's status and category.