PDS "Get Patient" should support If-None-Match request

GP System regularly poll for changes to the Patient record. Most of the time that PDS Record has no changes.

The PDS FHIR API sends back the "ETag" header when getting the Patient Resource (e.g. ETag: W/"4").

We would like to specify If-None-Match: W/"4" on a subsequent GET request for the same patient.
If it has not been modified (no new version), the PDS API should respond with HTTP/1.1 304 Not Modified (and an EMPTY payload).

[See https://www.w3.org/2005/MWI/BPWG/techs/CachingWithETag.html for an example of how this works.]

Benefits:
- Quicker / less resource intensive because less data being transferred needlessly.