[API] Adding APIs to delete specific versions of feature-store objects

[theSaarco]

Added REST API and DB implementation for deleting specific versions of feature-set and feature-vector objects. Versions are identified by a reference (either uid or tag), as usual.
APIs are also supported through the httpdb interface in delete_feature_vector and delete_feature_set.

Delete REST APIs supported now are:

DELETE /projects/{project}/feature-sets/{name}
DELETE /projects/{project}/feature-sets/{name}/references/{reference}

(and of course, same for feature-vectors)