multi: add zstd support

[jharveyb]

ZSTD is both faster and compresses at higher ratios than gzip, so it would be useful to have it as an option when rotating logs.

Further pro-ZSTD arguments here:

golang/go#62513

[jharveyb]

Closed in favor of #2 , with go.mod fixes - I didn't see a way to change the PR branch here.

[jrick]

I'm going to experiment with a different API, something like:

func (r *Rotator) SetCompressor(newCompressor func() io.WriteCloser, suffix string) {

It'll still default to gzip, but you should be able to pass in whatever other compressor and file suffix you want, and it won't pull in any additional dependencies that would otherwise go unused.

If/when zstd is added to the standard library, we can add a convenience method that calls it with the proper arguments to use zstd.