Add xsync.WorkGroup that is similar to https://godoc.org/golang.org/x/sync/errgroup,
but amends it with a bit better (imho) design where work context is
explicitly passed into worker (see also [1]). This mirrors sync.WorkGroup that we
already have at Pygolang side [2].

[1] https://github.com/golang/go/issues/34510
[2] https://pypi.org/project/pygolang/#concurrency