Extract `cli/clistat` to its own project

# Motivation

We have an internal library in coderd `cli/clistat` which we use in the `coder stat` command.
Extracting this to its own library would improve re-usability, and enable integration testing on multiple environments more easily.

This will provide an easier way to detect issues similar to the following:
- https://github.com/coder/coder/issues/15635
- https://github.com/coder/coder/issues/15779

As OOM and OOD notifications will end up consuming `coder stat` more widely, it will be important to have additional confidence in the underlying implementation.