add: config db-timezone-enabled for optional querying of timezones

To avoid repeated querying of `pg_timezone_names` every time schema
cache is reset, `Prefer: timezone` can be disabled by setting
`db-timezone-enabled = false`.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

test/io/configs/expected/jspath-str-op-dump5.config

@@ -18,6 +18,7 @@ db-root-spec = ""
 db-schemas = "public"
 db-config = true
 db-pre-config = ""
+db-timezone-enabled = true
 db-tx-end = "commit"
 db-uri = "postgresql://"
 jwt-aud = ""