Add --watch flag for generate command to regenerate files on changes

[fira42073]

buf generate --watch is a blocking call that uses https://github.com/fsnotify/fsnotify to watch filesystem changes and regenerate code.

Fix #2418

[CLAassistant]

All committers have signed the CLA.

[fira42073]

I've also bumped up the docker version from v27.0.0 to v27.0.01 to avoid the issue

go: downloading github.com/docker/docker v27.0.0+incompatible
go: github.com/bufbuild/buf/private/bufpkg/bufremoteplugin/bufremoteplugindocker imports
        github.com/docker/docker/api/types: reading github.com/docker/docker/go.mod at revision v27.0.0: unknown revision v27.0.0
go: github.com/bufbuild/buf/private/bufpkg/bufremoteplugin/bufremoteplugindocker imports
        github.com/docker/docker/api/types/image: reading github.com/docker/docker/go.mod at revision v27.0.0: unknown revision v27.0.0
go: github.com/bufbuild/buf/private/bufpkg/bufremoteplugin/bufremoteplugindocker imports
        github.com/docker/docker/client: reading github.com/docker/docker/go.mod at revision v27.0.0: unknown revision v27.0.0
go: github.com/bufbuild/buf/private/bufpkg/bufremoteplugin/bufremoteplugindocker imports
        github.com/docker/docker/pkg/jsonmessage: reading github.com/docker/docker/go.mod at revision v27.0.0: unknown revision v27.0.0
go: github.com/bufbuild/buf/private/bufpkg/bufremoteplugin/bufremoteplugindocker imports
        github.com/docker/docker/pkg/stringid: reading github.com/docker/docker/go.mod at revision v27.0.0: unknown revision v27.0.0

More context: https://bufbuild.slack.com/archives/CRZ680FUH/p1722779009632599

[fira42073]

Solutions inspired by:
https://github.com/air-verse/air/blob/master/runner/util.go#L259
https://github.com/fsnotify/fsnotify/blob/main/cmd/fsnotify/dedup.go

[bufdev]

Thanks for the PR. We don't have too much bandwidth to dive into this at the moment, but we'll get back to you. There are some issues with how the watch is handling inputs, and the separation of concerns with the os package, but we unfortunately can't dive into them at the moment.