KAFKA-14484: Decouple UnifiedLog and RemoteLogManager

[mimaison]

This introduces an interface, AsyncOffsetReader, to decouple UnifiedLog and RemoteLogManager.

This removes the last dependency UnifiedLog has on core so we can finally move it to the storage module.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[ijuma]

Do we think this interface is useful long-term or is this just a temporary thing while we move things around?

[mimaison]

Ultimately the goal is to also move RemoteLogManager to the storage module, so in theory we could remove it once that is done.
RemoteLogManager still depends on a bunch of core classes (I think we'll need to introduce a few interfaces to decouple it from ReplicaManager/Partition too) so it not quite ready to move yet, hopefully in the next few months.

I've got to admit I've not really thought a lot about it yet, but it may make sense to keep the interface to have a boundary between UnifiedLog and RemoteLogManager as even if they are both in storage, they'll be in different packages.

[mimaison]

Either way, I'll add some javadoc to the interface to describe its (current) role.