Allow entity lookup from entity value

[kzu]

If you're using the entities as DTOs too (i.e. over the wire by serializing to/from JSON),
it's convenient to be able to lookup a previously saved entity by using the entity at
hand, rather than having to use its partition/row keys (especially if those are annotated
properties and it's not obvious to the consumer which properties are used for each).

Since the repository internally already has an efficient way to access these properties,
which are used for the PutAsync operation, this would be a straightforward addition.