We bring in `containernetworking/plugins` for the contents of a single file,
which we use in a few places for running a goroutine in a specific network
namespace. This code hasn't needed an update in a couple of years, and a good
chunk of what we need was previously vendored into `client/lib/nsutil`
already.
Updating the library via dependabot is causing errors in Docker driver tests
because it updates a lot of transient dependencies, and it's bringing in a pile
of new transient dependencies like opentelemetry. Avoid this problem going
forward by vendoring the remaining code we hadn't already.
Ref: https://github.com/hashicorp/nomad/pull/20146
