Support for search parameters of type Composite

[lmsurpre]

Describe the bug
In DSTU2, there are only 2 or so composite SearchParameter types. As noted in https://github.ibm.com/watson-health-fhir-server/fhir-docs/blob/master/Conformance.md
In DSTU2, the composite SearchParameters are disabled (removed from the search-parameters.json.

Per the Spec https://www.hl7.org/fhir/valueset-search-param-type.html
composite | Composite | A composite search parameter that combines a search on two values together.

The SearchParameter drives a composite key from the parameter. In this case, when running an Observation resource, the entire resource is dropped into a single output value using the expression "expression" : "value.as(string)".

To Reproduce
In SearchUtil, there is a shortcircuit of the value public static <T> Map<SearchParameter, List<FHIRPathNode>> extractParameterValues(T resource) throws Exception {

• Disable the exception !SearchParamType.COMPOSITE.equals(type).
• Ensure that there is at least one value.as()

  {
    "fullUrl" : "http://hl7.org/fhir/SearchParameter/Questionnaire-context-type-quantity",
    "resource" : {
      "resourceType" : "SearchParameter",
      "id" : "Questionnaire-context-type-quantity",
      "extension" : [{
        "url" : "http://hl7.org/fhir/StructureDefinition/structuredefinition-standards-status",
        "valueCode" : "trial-use"
      }],
      "url" : "http://hl7.org/fhir/SearchParameter/Questionnaire-context-type-quantity",
      "version" : "4.0.0",
      "name" : "context-type-quantity",
      "status" : "draft",
      "experimental" : false,
      "date" : "2018-12-27T22:37:54+11:00",
      "publisher" : "Health Level Seven International (FHIR Infrastructure)",
      "contact" : [{
        "telecom" : [{
          "system" : "url",
          "value" : "http://hl7.org/fhir"
        }]
      },
      {
        "telecom" : [{
          "system" : "url",
          "value" : "http://www.hl7.org/Special/committees/fiwg/index.cfm"
        }]
      }],
      "description" : "A use context type and quantity- or range-based value assigned to the questionnaire",
      "code" : "context-type-quantity",
      "base" : ["Questionnaire"],
      "type" : "composite",
      "expression" : "Questionnaire.useContext",
      "xpathUsage" : "normal",
      "multipleOr" : false,
      "multipleAnd" : true,
      "modifier" : ["missing"],
      "component" : [{
        "definition" : "http://hl7.org/fhir/SearchParameter/Questionnaire-context-type",
        "expression" : "code"
      },
      {
        "definition" : "http://hl7.org/fhir/SearchParameter/Questionnaire-context-quantity",
        "expression" : "value.as(Quantity) | value.as(Range)"
      }]
    }
  },

Expected behavior
A new search is available using composite types and searches.

Additional context
Specific to Search and Persistence.

[lmsurpre]

Here's the full list of resources with one or more composite search params (sorted by the number of composite search params it has):

   8 "Observation"
   4 "MolecularSequence"
   2 "ValueSet"
   2 "TestScript"
   2 "TerminologyCapabilities"
   2 "StructureMap"
   2 "StructureDefinition"
   2 "SearchParameter"
   2 "RiskEvidenceSynthesis"
   2 "ResearchElementDefinition"
   2 "ResearchDefinition"
   2 "Questionnaire"
   2 "PlanDefinition"
   2 "OperationDefinition"
   2 "NamingSystem"
   2 "MessageDefinition"
   2 "Measure"
   2 "Library"
   2 "ImplementationGuide"
   2 "GraphDefinition"
   2 "ExampleScenario"
   2 "EvidenceVariable"
   2 "Evidence"
   2 "EventDefinition"
   2 "EffectEvidenceSynthesis"
   2 "ConceptMap"
   2 "CompartmentDefinition"
   2 "CodeSystem"
   2 "ChargeItemDefinition"
   2 "CapabilityStatement"
   2 "ActivityDefinition"
   1 "Group"
   1 "DocumentReference"

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