Provide an optional migration path for sites using files/images to media entities

Problem/Motivation

In #2825215: Media initiative: Roadmap → we mentioned that:

"When all the above is done, change Standard profile to ship with ER fields to Media entities instead of File/Image fields. Don’t force existing sites to migrate. They can keep using files as we don’t remove any of that from core. We can provide optional migration path later, but don’t require it until D9."

While not mandatory, existing sites might want to migrate to media entities as soon as possible, in order to take full advantage of the new api and functionalities.

Proposed resolution

We should provide a migration / upgrade path that would help existing sites move to the the new media-in-core paradigm.

Copying from #2930513-6: File/image field to media field migration → , @juampynr has already begun work on such a migration path in contrib: https://www.drupal.org/project/media_migration →

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD