feat: Support for dynamic hostnames

[Zambonilli]

Issue #, if available: https://github.com/awslabs/fhir-works-on-aws-deployment/issues/378

Description of changes: Add logic to optionally change the base hostname used on requests to either the configured API_URL environment in fhir-server-$stage lambda or the incoming request's hostname.

Checklist:

• [X] Have you successfully deployed to an AWS account with your changes?
• [X] Have you written new tests for your core changes, as applicable?

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #580 (4cd1ba4) into develop (9991809) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #580   +/-   ##
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  Coverage    94.84%   94.84%           
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  Files            1        1           
  Lines           97       97           
  Branches        14       14           
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  Hits            92       92           
  Misses           5        5           

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[nisankep]

FHIR Works on AWS has been moved to maintenance mode. While in maintenance, we will not add any new features to this solution. All security issues should be reported directly to AWS Security at [[email protected]] (mailto:[email protected]). If you are new to this solution, we advise you to explore using [HealthLake] (https://aws.amazon.com/healthlake), which is our managed service for building FHIR based transactional and analytics applications. You can get started by contacting your AWS Account team. If you are an existing customer of FHIR Works on AWS, and have additional questions or need immediate help, please reach out to [email protected] or contact your AWS Account team.