Add POST to [base]/[type]/_search to generated swagger/openapi

[lmsurpre]

Is your feature request related to a problem? Please describe.
https://www.hl7.org/fhir/search.html mentions that search can be invoked via a POST to [base]/[type]/_search.
Our FHIR server supports this, but this endpoint is not listed in our generated swagger / openapi definitions.

Describe the solution you'd like
Generated swagger / openapi definition should include this endpoint

Additional context
This endpoint is missing from the swagger that is generated for the various IGs as well. I brought it up at https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Search.20via.20POST

Suggestion for how to perform QA

1. For swagger: Go to https://editor.swagger.io/ and copy/paste generated swagger and verify it looks correct.
2. For openapi: Start up fhir-openapi server, (e.g. https://localhost:9443/openapi/ui/) and verify it looks correct.

[lmsurpre]

I navigated to https://localhost:9443/openapi/ui and confirmed that each resource endpoint now has an entry for POST to [base]/[type]/_search.

Additionally, with #1846, each resource also has a _search endpoint for each compartment they belong to.

I was able to invoke the POST from the Swagger UI and it seemed to work. However, when I try to add a body the tool seems to put each character of my body into its own form param:

[lmsurpre]

from Troy:

In newer releases of swagger-ui, there’s a checkbox to choose whether empty parameters are passed to the REST call. But I have no idea which version of the openapi ui liberty is using, or how to change it. That was a mystery to me.

I think its acceptable.