Operator API to configure session idle and absolute timeouts

[david-crespo]

Currently these values are hard-coded in the Nexus config to 1 hour and 8 hours, respectively. It would make a lot of sense to allow the operator to set these values dynamically.

omicron/smf/nexus/multi-sled/config-partial.toml

Lines 8 to 9 in e800a00

	session_idle_timeout_minutes = 60
	session_absolute_timeout_minutes = 480

[david-crespo]

We will likely want this configurable per-silo.