Add ETag to ObjectMeta

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

All object stores support a notion of an ETag that uniquely identifies a resource, we should support this

Describe the solution you'd like

The challenge lies in how to support this for the InMemory and LocalFileSystem stores. In lieu of a better alternative, as described here a hash of the last modification time is likely sufficient.

Describe alternatives you've considered

Additional context

[Weijun-H]

Can you confirm if this enhancement is still needed? If it is, I am available to work on resolving it.

[tustvold]

I think this would be a great addition, I will also work on #3928 to enable this