[FIX] export_bg: Improve export field handling and add external ID for import compatibility when exporting relational fields

[fw-bot-adhoc]

Forward-Port-Of: #388

[roboadhoc]

[fw-bot-adhoc]

@rov-adhoc @maq-adhoc cherrypicking of pull request #388 failed.

stdout:

Auto-merging export_bg/models/export_bg_mixin.py
Auto-merging export_bg/static/src/views/list_controller.js
CONFLICT (content): Merge conflict in export_bg/static/src/views/list_controller.js

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

you can use git-fw to re-do the forward-port for you locally.

⚠️ after resolving this conflict, you will need to merge it via @roboadhoc.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[rov-adhoc]

@roboadhoc r+ bump