Expose number of containers attached to a conmon-rs process

As part of the conmon-rs integration with podman, we need a way of querying conmon-rs to get the number of containers already attached to a single conmon-rs process.

This will help us determine whether to add a new container to an existing conmon-rs or to create a new conmon-rs process because we have reached the allowed limit.

Podman plans to keep a list of active conmon-rs processes so that we can go through those and query the number of containers attached to each one before making a decision.

Start by inspecting how conmon-rs tracks processes and the containers attached to each process, then review the stated Podman integration need for querying that state. Done means Podman can query a conmon-rs process and receive its current container count so it can enforce the attachment limit when choosing a process.

Written by the indexing model from the issue text.