when querying _history with _since we are not using fhir search parsing rules for the timestamp

[d0roppe]

Describe the bug
when adding _since or _before to the _history query you have to have a timezone and you have to have a full timestamp with or without miliseconds to have the query currently work. We should be able to follow fhir search parsing for the timestamp that defaults to the utz timezone and allows partial timestamps.

Environment
fhir main.

Expected behavior
defaults to the utz timezone and allows partial timestamps.

[lmsurpre]

The spec isn't very clear on this but, in the table of parameters for _history, they document that the type of the _since parameter is an "instant" and they link instant to https://www.hl7.org/fhir/search.html#date which does seem to imply that we should use the datetime model from fhir-search for this (rather than our FHIR model's DateTime support).

With that said, its less clear since they use the word "instant". One option is to ask for clarification at chat.fhir.org.

The _before parameter is something of our own invention, but that should work the same as _since for consistency.

Note: whatever we decide, it does NOT make sense to support search prefix values (lt, le, gt, etc) with these parameters.

[d0roppe]

A documentation change was added to state the the _before and _since are treated as instant data types and linked to the definition of instant data type in the spec. This clarifies what you need to provide in the timestamp to use _before and _since. Closing issue