Allow the x-configcat-sdkkey header on API and OFREP endpoints

[z4kn4fein]

Describe the purpose of your pull request

• Allow the x-configcat-sdkkey header on API and OFREP endpoints by listing it in the Access-Control-Allow-Headers CORS header.

Security (only if applicable)

• n/a

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.

[sonarqubecloud]

Quality Gate failed

Failed conditions
3.8% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud