Add a cagg config parameter to override the instance-level timescaledb.enable_tiered_reads GUC

[zilder]

This PR doesn't contain any API changes yet (to be discussed).

Refresh policy now includes or excludes tiered data based on policy's enable_tiered_reads config parameter regardless of the timescaledb.enable_tiered_reads GUC:

{
    "end_offset": "1 day",
    "start_offset": "6 months",
    "mat_hypertable_id": 43,
    "enable_tiered_reads": true
}