Add PATCH for runtime config

Summary

Implement PATCH on the /runtime-config endpoint. It should accept application/merge-patch+json.

Current Behaviour

Currently only PUT is implemented, making it hard to change a single variable.

Expected Behaviour

One useful example would be setting a banner automatically with a request like

curl -X 'PATCH' \
  'http://backend.localhost/api/v3/runtime-config/frontendConfig' \
  -H "Authorization: Bearer $SCICAT_TOKEN" \
  -H 'Content-Type: application/merge-patch+json' \
  -d '{
  "statusBannerMessage": "There will be a downtime",
  "statusBannerCode": "INFO"
}'

Details