ManagedEntry should be exported from a public path

[pocmo]

We use py-key-value-aio through FastMCP and implemented our own storage backend by subclassing BaseStore. When we updated the library to 0.4.x we noticed that ManagedEntry had moved and we had to update our import. In doing so we realised there is no public path for it.

ManagedEntry appears directly in the signatures of the abstract methods that BaseStore requires external implementors to override. Any custom store implementation must import it. We are currently importing it from key_value.aio._utils, which is a private module.

[strawgate]

Yeah, im not super sure I intended ManagedEntry to be public or folks to inherit from BaseStore quite yet but it's pretty obvious why that would be useful.

I'll take a look at this and see what we might be able to do