add errgroup sample

[jackielii]

What was changed

add errgroup sample

Why?

errgroup synchronise cancellation is a common pattern.

Checklist

2. How was this tested:

go run errgroup/errgroup.go

3. Any docs updates needed?

No other updates needed. Added README.md

[CLAassistant]

All committers have signed the CLA.

[cretz]

Thanks! Can I make a few requests? Firstly, can you split into the starter and worker CLI off as we do in other samples? Second, can you update the root repo README to point to this sample? Finally, can you add a test? Thanks!