Add cors headers to custom endpoint preflight requests

[lincmba]

Resolves [https://github.com/onaio/fhir-gateway-extension/issues/51]

Engineer Checklist

• [ ] 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.