[FEATURE] Support E-Tag for HTTP sync

[condorcet]

Requirements

Problem statement

Currently, "HTTP sync" fetches the body and decides whether to update the flags based on SHA changes.

This process is performed at every polling iteration, which may cause an overhead.

Proposal

HTTP source can support E-Tag.

If E-Tag presented in response headers by server, then HTTP sync can add If-None-Match with the value of E-Tag in headers for further requests.
Server will return response with actual body, only if E-Tag of the content has changed (304 Not Modified otherwise).
This will help to avoid blank requests, when nothing changed in HTTP source.

[ChihweiLHBird]

I can try working on this if no one else started the work.

[beeme1mr]

Sure, that would be great. Thanks @ChihweiLHBird