Support for searching references by identifier

[lmsurpre]

From https://www.hl7.org/fhir/search.html#reference:

References are also allowed to have an identifier. The modifier :identifier allows for searching by the identifier rather than the literal reference:

GET [base]/Observation?subject:identifier=http://acme.org/fhir/identifier/mrn|123456
This is a search for all observations that reference a patient by a particular patient MRN. When the :identifier modifier is used, the search value works as a token search. The :identifier modifier is not supported on canonical elements since they do not have an identifier separate from the reference itself.

Chaining is not supported when using the :identifier modifier, nor are chaining, includes or reverse includes supported for reference elements that do not have a reference element.

[lmsurpre]

We think we're already handling the extraction side of this one, so it should be just a matter of updating our query generator to look in the TOKEN_VALUES_V view with the identifier system+value instead of the reference value.

[lmsurpre]

Currently, we don't even extract the identifier field of reference elements, so that would be step 1.

[prb112]

Completed QA. Looks good and behaved as expected.