Distribute a systemd `.service` file with the new CAS server

Background

Since the new CAS-based artifact server is an actual long-running daemon, we can sensibly support using it with common server infrastructure.

We could make a start by distributing the server with a systemd .service file - these are easy to write, but defaults often don't change much between servers.

Task description

• Find an a sensible set of default configuration options
• Produce a working systemd buildstream-cache.service file

Acceptance Criteria

Distribute a systemd service alongside the CAS server that makes deploying the artifact share as simple as possible on a normal linux host.