Unify handling of _type parameter across system-search, system-history, $everything and $export

[lmsurpre]

Is your feature request related to a problem? Please describe.
Currently, our system-history implementations accepts multiple instances of the _type parameter and combine them into a single list.

Conversely, our system-search implementation takes just the first one.

In a recent version of the bulkdata spec, they relaxed the cardinality and so multiple instances of this parameter should be supported there as well. We have a separate issue, #3081, for that one.

Describe the solution you'd like
across these 4 endpoints, we should have common behavior for this parameter

Describe alternatives you've considered

Acceptance Criteria

1. WHEN a search like GET [base]?_type=Patient&_type=Condition
   THEN the values of the _type parameters are combined
   AND the result matches GET [base]?_type=Patient,Condition

And verify this same behavior when:

1. system-history is invoked with multiple _type query params
2. $everything is invoked with multiple _type query params
3. $export is invoked via GET with multiple _type query params
4. $export is invoked via POST with multiple _type params in the Parameters body

And finally, verify that when $export is invoked via POST, none of the query parameters are used (only the Parameters from the body).

Additional context
We should probably document the behavior in Conformance.md as well, since this differs from how we handle other "return parameters" (#3318 )

[lmsurpre]

The $export operation has some special handling for the _types parameter where it checks in both the request url AND the Parameters object. I'd like to try normalizing the two so that downstream code can look in a single spot for request parameters. I think we already do something like that for extended operations that are invoked via GET.

[lmsurpre]

Based on the last comment, I added this line to the acceptance criteria:

And finally, verify that when $export is invoked via POST, none of the query parameters are used (only the Parameters from the body).

That was done so that its consistent with our other operation implementations (and so that it matches the fhir-smart logic for controlling access). That can be revisited under #3480

[d0roppe]

Verified that specifying _type=, or _type=&_type= are handled the same for Search, History, $everything, and Bulk Data Export as requested in the Acceptance Criteria.