Allow app devs to retrieve a list of keys in a bucket

[adamfowleruk]

Please start with a use case description for a USER of this work
[Who] As an application developer
[What] I need to list the keys in a bucket
[Value] In order to fetch only the keys in a particular bucket, reducing the number of DB queries I execute

Describe the solution you'd like
New function on KeyValueStore: getBucketKeys(const std::string& bucketName)

Describe alternatives you've considered
Manually going through keys is impossible - no key metadata returned, and no listAllKeys() function

Additional context
Implementing this will allow for a fair comparison of the performance of each store vs using a BucketQuery.