As a quick follow-up to #3124 and in response to
#3124 (comment),
this adds a `clean` config key to `v2` `buf.gen.yaml`, for example:

```yaml
version: v2
clean: true
plugins:
  - local: custom-gen-go
    out: gen/go
    opt: paths=source_relative
    strategy: directory
  - protoc_builtin: java
    out: gen/java
```

When running `buf generate` with the above configs, the outs set to each
plugin
(e.g. `gen/go` and `gen/java`) will be removed before code generation is
run.

If `buf generate --clean` flag is set, then that will always take
precedence, even if
`clean: false` in the configuration. And likewise, if `buf generate
--clean=false`,
and `clean: true` in the configuration, then we would not delete the out
directories.

---------

Co-authored-by: Oliver Sun <[email protected]>
Co-authored-by: Oliver Sun <[email protected]>
Co-authored-by: bufdev <[email protected]>
Co-authored-by: bufdev <[email protected]>