Add support for different references

I would like to have the possibility to add own references in the build.yml for fields that are not in ecs, but standardized across my set of log sources. Those fields will be defined in the same yaml structure, but need to have a different name.

The fact that this needs to be **ecs** seems to be hardcoded at 2 different places.


Start by locating the two places where ecs is hardcoded and reviewing how references in build.yml are parsed. Trace the existing ECS-shaped YAML path and determine how an alternate name should be represented. Done means build.yml can declare standardized non-ECS references with a different name while preserving existing ECS references.

Written by the indexing model from the issue text.