/push API partial writes

**Is your feature request related to a problem? Please describe.**
Is it possible to have the /push API accept a payload of a subset of the fields of a defined feature? Or does the need for this imply that the feature is not correctly modeled abstractly?

**Describe the solution you'd like**
Our use case of feast would like to see the ability to group fields under a single feature but allow for partial subsets of fields to be updated at a time.