Add cors headers to custom endpoints

Engineer Checklist

[x] I have written Unit tests for any new feature(s) and edge cases for bug fixes
[x] I have run mvn spotless:check to check my code follows the project's style guide
[x] I have run mvn clean test jacoco:report to confirm the coverage report was generated at plugins/target/site/jacoco/index.html
[x] I ran mvn clean package right before creating this pull request.

onaio / fhir-gateway-extension