Align reportType parameter values with MeasureReport.reportType element

[brynrhodes]

The implementation should support "individual" and "summary" as reportTypes, as those are the values of the actual reportType element in the MeasureReport resource.

Note that this is currently oustanding as a TC to the DEQM, and that implementations should continue to support the current "subject" and "population" values for backwards compatibility, so this is a low-priority issue.

https://jira.hl7.org/browse/FHIR-51857

[brynrhodes]

Note as part of this that when we pass an invalid reportType to the $evaluate operation, the result is a 5XX error, whereas it seems like this ought to be a 4XX level error, since it is user-correctable?