Move ajax callback to WebformPostcodeAPI element

Problem/Motivation

The current ajax callback replaces part of the form with new form elements. However, the new form elements are not part of the form build, and are not accepted when submitting the form.

Proposed resolution

Move ajax callback to WebformPostcodeAPI element, so the updated form elements are accepted when the form is submitted.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

The updated form elements are accepted when the form is submitted.

API changes

None.

Data model changes

None.