Search parameter filtering fails for search parameter which has multiple base resource types

[michaelwschroeder]

Describe the bug
When filtering search parameters, we filter based on resource type. However, rather than using the resource type specified on the search, we're grabbing the first element in the SearchParameter.base field. So if a search parameter has more than one base and the first base element does not match the resource type specified in the search, then we filter on the wrong resource type and can possibly fail.

To Reproduce
Steps to reproduce the behavior:

1. In fhir-server-config.json, configure a search parameter that has multiple base resource types (for example the medication search parameter) to be valid on only one of those resource types (for example, MedicationRequest).
2. Execute a search request using the configured search parameter against the configured resource type: MedicationRequest?medication=xxx

Expected behavior
The search should run successfully.

Additional context
Add any other context about the problem here.

[lmsurpre]

I submitted a MedicationRequest example from US Core that has the following:

  "medicationReference" : {
    "reference" : "#med2",
    "display" : "Nizatidine 15 MG/ML Oral Solution [Axid]"
  },

Then I submitted a request for [base]/MedicationRequest?medication=%23med2 and it successfully returned the resource.