v2 Config: Allow outputs to be specified with inputs

[bhollis]

The v2 config allows me to specify inputs in buf.gen.yaml, which would be great if all of my code was meant to be generated to the same place. Instead, what I want is to use the same plugins, the same relative output paths, but different output directories per input. If we could specify outputs along with inputs, we could get down to a single buf generate invocation - instead, we still have to call it once per input.

[doriable]

This is pretty commonly requested, we have an issue centralising and tracking the discussions on this #3060, we will definitely be discussing this internally! Thanks for putting up this discussion, it helps us prioritise and gather these concerns :)