[feature] Support extensible export of ForeignKey relations in export_users management command

[pandafy]

Is your feature request related to a problem? Please describe.
Currently, the export_users management command in openwisp-users does not support exporting ForeignKey relations from related objects. While there is special handling for the organizations field, this approach is not sufficient when there is a need to include fields defined in other modules (outside the core openwisp-users domain).

openwisp-users/openwisp_users/management/commands/export_users.py

Lines 66 to 70 in 2ef104e

	elif field == "organizations":
	organizations = []
	for org_id, user_perm in user.organizations_dict.items():
	organizations.append(f'({org_id},{user_perm["is_admin"]})')
	data_row.append("\n".join(organizations))

This limitation is highlighted by the upcoming model changes discussed in openwisp/openwisp-radius#692, where the RegisteredUser model is being updated to support a multi-tenant structure. Specifically, RegisteredUser will now have a ForeignKey to the User model. As a result, it is necessary to provide generic mechanisms or extension points to support exporting custom fields from objects linked through ForeignKeys.

Additional context
This requirement is motivated by openwisp/openwisp-radius#692, where the RegisteredUser model changes will introduce a ForeignKey to the user. Being able to customize how ForeignKey fields from related modules are exported will greatly facilitate multi-tenant and modular setups in OpenWISP.