[libartifact] Introduce an event channel in `ArtifactStore`

[nimdrak]

Proposal

I propose adding an event channel to the ArtifactStore struct

Motivation

The primary goal is to provide a mechanism for ArtifactStore to broadcast events related to its operations, such as artifact push or pull actions.
It is an issue in podman containers/podman#27260 (comment)

Benefits

This addition offers two significant advantages:

1. Enables Real-time Event Consumption: Clients using ArtifactStore can subscribe to this channel. This allows them to receive immediate notifications and react to artifact events as they happen.

2. Decouples Event Logic: This design simplifies the client's responsibilities. Clients are decoupled from the specifics of event generation. They can simply consume events directly from the provided channel.

Reference

This is the same architecture as libmage.Runtime

• runtime channel: https://github.com/containers/container-libs/blob/main/common/libimage/runtime.go#L70
• runtime client: https://github.com/containers/podman/blob/2e738b9b871f98930d34ca97bc0eb19a324193ea/libpod/runtime.go#L708