Add support for buf.gen.yaml initialization

[remyleone]

when coming to a new repository where no buf.gen.yaml is declared, I get the following error:

$ buf generate
Failure: buf.gen.yaml: does not exist.

Could it be possible to have a flag such as buf generate --init or something similar that generates a stub for buf.gen.yaml. In particular, that could be a nice place to show the official generators but also useful generators coming from the buf community

[bufdev]

We'll consider this and get back to you. Note for future: likely add this to buf config.