Make it possible to ignore `buf.yaml` in the `buf_format` rule

[ash2k]

I have a buf.yaml file in the repo, but it only lists some of the proto files/dirs (for unrelated reasons). I'd like to use the buf_format Bazel rule to format all the proto files in the repo, not only what is listed in buf.yaml. There is currently no way to do what I'd like to do.

Would be good to make it possible to either specify a custom config or ignore the existing config (my preferred option). Or maybe both.

Thank you!