Add `validateForm` mutation

### What problem does this address?

This feature was added to GF 2.6.4, and can serve to provide background async validation to cut down on submission times for large fields.

https://docs.gravityforms.com/validating-forms-with-the-gfapi/

### What is your proposed solution?

..

### What alternatives have you considered?

_No response_

### Additional Context

_No response_