Custom fields API release

Introducing brand new sub-section under the Users section: Custom Fields. The ability to manage the custom fields settings associated with the account

Added the following new endpoints:

• Create Custom Fields: Create individual or multiple custom fields under a specific category.
• Get Custom Fields: Retrieve custom fields associated with the account, with the option to filter from relevant categories.
• Update Custom Field: Modify existing custom field properties.
• Delete Custom Field: Remove an existing custom field from the system.
• Create Custom Field Categories: Establish a new custom field category with a specified name.
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