Add created and changed base fields to external entities

Patched attached shows how the implementation would look like.
Is this something we would consider useful?

Questions!
Why not letting the external entity admin create the external entity type and add those fields manually if needed?
Why should they be there for any external entity type?

My question was primarily for EntityChangedInterface, which is provided by Drupal core and in use by some (sub)systems/contrib modules. I found out about this in a custom project of mine, where a piece of generic code didn't have the expected behavior in the case of external entities (e.g. if ($entity instance of EntityChangeInterface) { ... }).

I guess I can agree with the created-property, there doesn't seem to be an interface for this (while some core-entity types do implement it in their own interface, though).

If this is not desired, I guess I'll just have to extend the ExternalEntity class for my use-case.

I'll let v2 developers judge for v2. For v3, I won't integrate that part for now. It is something that can be added later without breaking things I think (just a new feature). So, if it appears in the future as a really need feature, I'll add it then. ;)

Created and changed properties would be useful for record keeping.

I was thinking changed could be useful for time based syncing (IE update every entity that has not been synced in the last 24 hours).

Some more documentation on how the caching works would be helpful. I can see that it's stored in a table, but not sure about the logic around when it gets updated.