suggestion: enable simple "cluster age" queries

Our current API accepts "cluster age" filter criteria as follows:
- created before a certain date
- created after a certain date
- checked in before a certain date
- checked in after a certain date

From a convenience standpoint, it would be nice to enable just one, or two, or three of those from the API consumer (more closely matching a human semantic expression), e.g., "clusters that have checked in since last week". This will require either overloading the existing "cluster age" API interface to allow the ommission of certain filter inputs, or to create novel API interfaces for simpler requests.