Support key mapping that uses a key field rather than a column

Once the partition strategy for HBase is defined independent of column mappings, the "key" mapping should indicate that the field can be populated using the value stored in the entity's key. We will need to validate that the partitioner for the given field is an identity partitioner.

For backward-compatibility, we should continue to use the key mappings to produce an identity partition strategy when there isn't one from the metadata provider.