Support static configuration of external FHIR base URL (in favor of originalRequestUriHeader)

[d0roppe]

Is your feature request related to a problem? Please describe.
The current implementation of the originalRequestUriHeader is a little more complicated than it should be. we can pass in a full uri including search criteria, and the fhir server needs to figure out what it should replace.

Describe the solution you'd like
I propose that the use of the originalRequestUriHeader should expect the [base] part of a URI, and replace the [base] part of the result url's with the value entered no matter what was entered, as it is a user provided header for text replacement of the [base] part of the url only. this will keep the code more simple and will result in predictable replacement.

Describe alternatives you've considered
no change from current implementation

Acceptance Criteria

1. If implemented test that only the [base] part of the result url is affected and is directly replaced with what is passed in on the header.

Additional context

[lmsurpre]

Suggestion is to continue supporting the current method which is to get this value from an incoming header IN ADDITION to this new method.
However, the two are a little different because the header will contain the full url (with or without query parameters) whereas this config option would be just the BASE url. (typically https://host:port/fhir-server/api/v4 for us)

[d0roppe]

This is working using the "externalBaseUrl" and the text entered in the value is directly replaced. So as long as you provide a valid [base] url it is working just fine. Closing issue.