"Field field_* is unknown" exception when importing

I ran into an issue trying to import custom fields via an XML source file (in that, the latest dev version was totally ignoring custom fields).
As a result, I wrote this patch that currently includes a lot of debug logging to check and log what custom fields were matched with the taxonomy, and which will be omitted.

The patch could use a review/testing and can then be further refactored to:
1. achieve the form error messages suggested here.
2. include instructions/documentation about how to configure your XML file to support custom fields (namely, the keys must match machine names of fields) which would address https://www.drupal.org/project/taxonomy_import/issues/3392992 📌 Document importing custom fields Active
3. testing and refactoring for CSV to parse/respect custom fields as well.

For my use case, I realized our customer did not have unique term names, and as a result parent term matching failed based solely on "name".
Since the import processes in order, and our source data assigned parents based on the last known match by term name (newest added term with that name), I added a field to the import form to force creating new terms for each row of data and updated parent matching to be last match identified (which still works in the case all term names are unique).

In addition, I cleaned up some of the debug logging. Updated patch attached for testing and review.

As an update to the refactoring next steps I recommended in the prior comment:
1. In order to achieve the form error messages suggested originally in this ticket here, it assumes the XML file is uniform for every row (my source data omitted blank fields)- hence the parsing of custom fields on a per record/row basis in the patch.

2. include instructions/documentation about how to configure your XML file to support custom fields (namely, the keys must match machine names of fields) which would address https://www.drupal.org/project/taxonomy_import/issues/3392992 📌 Document importing custom fields Active

3. testing and refactoring for CSV to parse/respect custom fields as well.