Customizable models in ModelDB CI workflow in PRs

[JCGoran]

When running CI jobs in a PR, it's currently only possible to run the ModelDB CI workflow with all of the models selected (by applying the nrn-modeldb-ci-nightly label to a PR). It is possible that a given PR only impacts a handful of models, and having the ability to run the workflow with only a customizable subset of the models would greatly speed-up the CI job.