feat: Allow journald directory to be overriden

[jessedavis]

The default directory used by the journald receiver (/var/log/journal) is governed by the Storage= setting in journald.conf (usually Storage=auto). Some OSes seem to default this to /var/log/journal; others, such as Rocky Linux, default this to /run/log/journal. If the directory is not created by the administrator and used by journald, the following error is seen:

2025-10-10T14:25:03.226Z	error	journald/input.go:164	Received from journalctl stderr	{"resource": {"service.instance.id": "28da9c50-9c3e-4d93-b3f4-0ba1035c2671", "service.name": "otelcol-sumo", "service.version": "0.130.1-sumo-0-9cd0cff3fd63f4db526097a355ef9672f0956027"}, "otelcol.component.id": "journald", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "operator_id": "journald_input", "operator_type": "journald_input", "stderr": "Data from the specified boot (+0) is not available: No such boot ID in journal\n"}
github.com/open-telemetry/opentelemetry-collector-contrib/pkg/stanza/operator/input/journald.(*Input).runJournalctl.func1

The administrator could manually create the directory, or the directory could be specified via the chart.

Checklist

• Documentation updated
• Template tests added for new features