SearchParameterCanonicalizer Does Not Account for Search Parameters for Custom Resources Types When Converting DSTU2&3 Into RuntimeSearchParam

[TynerGjs]

Describe the bug
The SearchParameterCanonicalizer does not correctly convert DSTU2 and DSTU3 custom resources search parameters into RuntimeSearchParam.

Expected behavior
Extensions with EXTENSION_SEARCHPARAM_CUSTOM_BASE_RESOURCE and EXTENSION_SEARCHPARAM_CUSTOM_TARGET_RESOURCE are correctly recognized as base and targets for the search parameter.
Instead, these extensions are ignored and only the resources in the base and target fields are used to create the search parameter

Environment (please complete the following information):

• HAPI FHIR Version rel_6_6

Additional context
The expected structure of search parameters for custom resource types is:

{
  "resourceType" : "SearchParameter",
  ...
  "base" : ["Resource"],
  "type" : "reference",
  "target" : ["Resource", "Condition"],
  ...
  "extension" : [{
    "url" : "http://hl7.org/fhir/tools/CustomBaseResource",
    "valueCode" : "ThingA"
  },{
    "url" : "http://hl7.org/fhir/tools/CustomTargetResource",
    "valueCode" : "ThingB"
  }]
}