Idea: support augmentation of generated YAML

While it's super handy generating workflows from SBT, Github are adding features to actions all the time and that presents a binary choice when wanting to use this plugin or a certain Github actions feature, if said feature hasn't been implemented here. An example of this is #7.

Kubernetes have the Kustomize tool, that allows for YAML fragments to be merged with the output of a template or base file. I was wondering if this might be a good idea for this plugin? I realise that this would be very much a "use at your own risk" feature, as people could end up breaking what would otherwise be a working workflow, but it would enable usage of the entire feature set from Github actions.

No files, tests, or entry points are named. Start by locating the SBT plugin code that generates GitHub Actions YAML and review how generated workflows are assembled. Define the augmentation format, merge behavior, and validation expectations before implementation; done should allow custom YAML fragments to extend generated workflows without losing existing output.