An ftl command for generating module stubs for external libraries to use

[alecthomas]

It's currently impossible to create libraries that can operate on FTL types, which results in a lot of duplication. I'm not sure if this is the final solution, but I think having a command that can generate FTL types from a set of existing modules or the cluster would be good. Basically identical to ftl build's code-generation steps, but generating stubs in normal Go or Kotlin packages.

Let's discuss further next week.