New config to add required launcher fields

[Bubballoo3]

While testing the functionality of launcher_default_fields, I was surprised to learn that unlike the required fields of cluster and scripts, custom default fields can be deleted from the launcher after creation.

While this isn't necessarily an issue by itself, and is accurately reflected in the naming default instead of required, I had imagined the purpose of this config being for centers to add on any attributes that they specifically require, like how OSC requires accounts on all jobs. For this use case, I don't see the value in letting users delete these additional fields, and wonder why a center would add optional attributes to the default list in the first place (that is, default values that you would want to delete), though I can see the argument that this is mostly to prevent support tickets and that under the current framework the user would have to intentionally delete the field to break the launcher.

I think I would like to see us rename the launcher_default_fields to launcher_required_fields to ensure that launchers are always valid to the scheduler provided that this config is set up correctly, and it is much easier to do this before 4.1 is released than try to change it later.