Added support for product category search for coupons

[tanmayjay]

All Submissions:

• My code follow the WordPress' coding standards
• My code satisfies feature requirements
• My code is tested
• My code passes the PHPCS tests
• My code has proper inline documentation
• I've included developer documentation (optional)
• I've added proper labels to this pull request

Changes proposed in this Pull Request:

Part of https://github.com/weDevsOfficial/dokan-pro/pull/2099

Changelog entry

Update: Added support for product category

PR Self Review Checklist:

• Code is not following code style guidelines
• Bad naming: make sure you would understand your code if you read it a few months from now.
• KISS: Keep it simple, Sweetie (not stupid!).
• DRY: Don't Repeat Yourself.
• Code that is not readable: too many nested 'if's are a bad sign.
• Performance issues
• Complicated constructions that need refactoring or comments: code should almost always be self-explanatory.
• Grammar errors.

FOR PR REVIEWER ONLY:

As a reviewer, your feedback should be focused on the idea, not the person. Seek to understand, be respectful, and focus on constructive dialog.

As a contributor, your responsibility is to learn from suggestions and iterate your pull request should it be needed based on feedback. Seek to collaborate and produce the best possible contribution to the greater whole.

• Correct — Does the change do what it’s supposed to? ie: code 100% fulfilling the requirements?
• Secure — Would a nefarious party find some way to exploit this change? ie: everything is sanitized/escaped appropriately for any SQL or XSS injection possibilities?
• Readable — Will your future self be able to understand this change months down the road?
• Elegant — Does the change fit aesthetically within the overall style and architecture?