Replace sql-exporter with Kopf timer

[MarkusH]

At this point, we're deploying a sql-exporter sidecar container to the crate container. The sql-exporter provides a way to monitor the cluster availability. In the future, it might be a good idea to replace the sidecar container with a Kopf timer. Something along these lines:

@kopf.timer(API_GROUP, "v1", RESOURCE_CRATEDB, interval=15.0)
async def cluster_health(namespace: str, name: str, logger: logging.Logger, **kwargs):
    core = CoreV1Api()
    host = await get_host(core, namespace, name)
    password = await get_system_user_password(namespace, name, core)
    try:
        async with get_connection(host, password) as conn:
            async with conn.cursor(timeout=0.5) as cursor:
                await cursor.execute("SELECT count(state) FROM sys.shards")
                await cursor.fetchone()
    except Exception:
        prometheus._clusters_status.set({"namespace": namespace, "name": name}, 0)
        logger.error("Cluster is not responsive")
    else:
        prometheus._clusters_status.set({"namespace": namespace, "name": name}, 1)

Depends on #45