Add functionality to get /slabs

[faisal-memon]

Proposed changes

Adding support for /slabs API endpoint so we can monitor shared memory zone usage.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[Rulox]

lgtm, a small suggestion on a error string and !

[pleshakov]

thanks @faisal-memon !
👍

[Dean-Coakley]

Thanks @faisal-memon !

In a separate PR I'd like: https://github.com/nginxinc/nginx-plus-go-client/blob/9a147f564d6a7bd5a1e023f1ca93728c2dec8a2c/tests/client_test.go#L580 to be extended to include some basic test coverage of slabs & slots stats.

Let us know if you want to add it. - If not, we'll add it.