ETag and/or Last-modified headers in API

[tuxpiper]

Which user group(s) are the primary audience for this feature/enhancement.
All deployments, but specially those with high-traffic/concurrency.

Is your feature request related to a problem? Please describe.
Currently the REST API is not tagging its resources with ETag or Last-Modified headers.

This makes prevents taking advantage of caching re-validation mechanisms and conditional updates (i.e. fail when

Describe the solution you'd like

• Implement ETag (preferably) headers for primitive resources (i.e.: Post, Tag, Form, User, etc)
• Implement Last-Modified headers for searches or other complex resources (i.e.: Survey)
• Properly implement HEAD methods

Describe alternatives you've considered
n/a

Additional context

• https://www.kennethlange.com/avoid-data-corruption-in-your-rest-api-with-etags/
• https://www.youtube.com/watch?v=Cff0OCIDWoc
• https://fideloper.com/api-etag-conditional-get

Aha! Link: https://ushahiditeam.aha.io/features/PROD-888