Add ClientWrapper class

Experimental implementation of https://gitlab.com/gitlab-org/release-tools/-/issues/412.

When the client_wrapper feature is enabled, the existing GitlabClient-like classes will internally use a wrapped gem client instance, which should be entirely transparent.

This will allow us to test the new behavior, while being able to instantly turn it off, before we eventually start removing things from the old client classes before eventually removing them entirely.