`IContainersClient` needs to expose the internal hostname and default bridge network name (maybe?)

[bwateratmsft]

For example, host.docker.internal for Docker and host.containers.internal for Podman.

Required for microsoft/vscode-containers#258.

[bwateratmsft]

Some incomplete changes are on https://github.com/microsoft/vscode-docker-extensibility/tree/bmw/pythonPodmanDebug.