Summary:
Exposing new API in CustomSectionAPI for column mapping.
The custom widget can call configure method (or use a ready method) with additional parameter "columns".
This parameter is a list of column names that should be mapped by the user.
Mapping configuration is exposed through an additional method in the CustomSectionAPI "mappings". It is also available
through the onRecord(s) event.
This DIFF is connected with PR for grist-widgets repository https://github.com/gristlabs/grist-widget/pull/15
Design document and discussion: https://grist.quip.com/Y2waA8h8Zuzu/Custom-Widget-field-mapping
Test Plan: browser tests
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3241
