Be able to add a header to all files generated

[markmandel]

Is your feature request related to a problem? Please describe.

My use case is being able to apply an Apache 2 header to all generated files in an open source project - as well as being able to specify in a top level header that the file is generated and shouldn't be edited by hand.

Describe the solution you'd like

I'd love a --prepend-header-file option (or similar) to the project which I can point to a file that contains any header content I want to add to all files that are generated.

Describe alternatives you've considered

I could do this after the fact in my own scripting - but I expect proto-gen validate will then fail, as if I add the header the source will be different from what is generated.

Additional context

Generated code in https://github.com/googleforgames/quilkin/tree/main/crates/quilkin-proto/src should all have apache headers.