Allow to avoid translation filtering

This is not only an issue for the D7 migration.
For D9, there is also a case when you need to migrate all values regardless of language.
This need arises when the default language of the entity may differ from the default language of the site.
In this case, the easiest and most effective way is to migrate all files to the media entity, regardless of the language of translation.
Since this solution is universal, I applied it to all cases when we migrate data with translations.

How it works for now:
- source_lang_code is required for the source plugin
- langcode could be null - it means that we will migrate all files without translation filter
- when langcode is null then source_lang_code will be used for migration rows

P.S. Keeping a ticket in review status to allow someone else to review it. If the task remains unchanged for 2 weeks, the solution will be automatically added to the dev branch.

Automatically closed - issue fixed for 2 weeks with no activity.

For anyone else who is updating their D9 site to D10 and ran into an error running 'drush migrate:status' :

source_langcode configuration key should not be an empty

Your existing migrations should be updated to include the source_langcode key in the source section.