Until we reach conclusions and good design on how we're going to handle "extends" in the long run for configuration, we can implement a simple include mechanism for yaml files:

• the spec allows us to create custom "tags" (e.g !include foo/bar.yaml)
• we can benefit from the work being done with MAGNOLIA-6128 et al such that includes could reside anywhere
• included files wouldn't suffer from indentation issues like a pre-processor could cause (i.e the include is parsed into a map and added as a value to the map-under-parsing)
  • as a consequence, references&anchors won't be useable between a "main" file and its includes