feat(dgw): return disk space available for recordings

[CBenoit]

The total and available space used for storing recordings is now returned in the heartbeat response.

If the system does not support this operation, the fields are excluded from the response.

Issue: DGW-100

Example using curl

$ curl -H "Accept: application/json" -H "Authorization: Bearer $(./tools/tokengen/target/release/tokengen --provisioner-key ./config/provisioner.priv.key scope gateway.heartbeat.read)" http://127.0.0.1:7171/jet/heartbeat

{"id":"aa0b2a02-bbbb-aaaa-bbbb-03c6391b86fb","hostname":"localhost","version":"2024.1.5","running_session_count":0,"recording_storage_total_space":737616650240,"recording_storage_available_space":199578578944}