omdb: Add one-shot query exec to oxql subcommand

[wfchandler]

In some cases, we may execute an oxql query as part of an ad hoc shell script, or have capture the queries executed in the shell history. The mandatory reedline shell used by the oxql subcommand prevents this.

Add a --execute flag to provide a one-shot execution option to the oxql subcommand.

[wfchandler]

Tested out on dogfood:

root@oxz_switch0:/tmp# ./omdb-oxql-exec oxql -e 'get sled_cpu:cpu_nsec | filter timestamp > @now() - 1m' 2>/dev/null | head

sled_cpu:cpu_nsec

 cpu_id: 9
 rack_id: de608e01-b8e4-4d93-b972-a7dbed36dd22
 sled_id: 781ab3c9-ff3c-428a-9f3e-895a1e86a111
 sled_model: 913-0000023
 sled_revision: 1
 sled_serial: BRM13250012
 state: user

[bnaecker]

Nice, thanks!