[BUG] Getting a network by ID does not work

See repro test [here](https://github.com/nlacasse/podman-py/commit/856b3690d0cd79b6c3935722ae9930c84ed157f6).

When we create a new network, podman returns its `id`. However, the [`Network.id`](https://github.com/containers/podman-py/blob/main/podman/domain/networks.py#L31-L40) method tries to return `self.attrs["Id"]`, but that always fails, so it falls back to returing a synthetic `id` based on the network name. If we pass this `id` to `network.get()`, the call fails, since Podman does not know any networks with that id.

Lowercasing `Id` -> `id` in [this line](https://github.com/containers/podman-py/blob/main/podman/domain/networks.py#L34) fixes the issue, and causes `Network.id` to return the correct `id` assiged by Podman. But I'm not sure if that's the correct fix.