Support Pods fields in REST API as params for meta query

[JoryHogeveen]

Description

This PR will add basic support for meta based REST API queries when passing Pods fields as parameters.

Related GitHub issue(s)

Fixes #7210

Testing instructions

1. Create CPT (or use Post)
2. Add a custom field
3. Enable REST API
4. Query field: example.com/wp-json/wp/v2/{CPT}?{FIELDNAME}={VALUE}

Changelog text for these changes

PR checklist

• I have tested my own code to confirm it works as I intended.
• My code follows the WordPress Coding Standards.
• My code follows the WordPress Inline Documentation Standards.
• My code includes automated tests for PHP and/or JS (if applicable).

[what-the-diff]

PR Summary

• Introduction of the query_fields function in PodsRESTFields class
  This modification brings in a new function called query_fields. This function comes in handy in refining the parameters for a REST request depending on the field values given. In simpler terms, it curtails through unnecessary data in a request, helping the program to process only the specific, relevant pieces of information.

• Inclusion of a filter hook named rest_{pod_name}_query
  Another function called rest_{pod_name}_query has been introduced, which allows us to apply the above query_fields function to a specific category (or 'pod' in programming language). Essentially, it provides a way for integrating the filtering feature with specific parts of our system.