Add a "Last Updated" timestamp/epoch in SDK API responses

[dabeeeenster]

Overview

There are situations where it would be helpful for SDKs/clients to know if any flags have changed between two requests. There are a couple of different ways I can think of achieving this:

• Add an epoch/last updated timestamp in the response for the Environment as a whole
• Add an epoch/last updated timestamp in the response for each Flag within the Environment

Or possibly include both.

Concerns

• Are there any issues including this data in public endpoints?
• Updates to Segment definitions would probably necessitate an update in the timestamp/epoch