Use built in zstd for compression when provided

[mattmatters]

For >=v28 of Erlang, Zstd is part of Erlang's stdlib. This greatly simplifies dependency requirements in Req.

I took some great pains to match how ezstd is called and matching its return type. It did prove to get a little ugly though to prevent compiler type warnings however.

Hopefully in several years this can be built with the expection zstd always exists.

[wojtekmach]

Thank you. The PR looks good to me. I need some more time to figure out if it's worth it to keep support for ezstd since it will be less need of that going forward.