[API] add gracePeriod one of the API endpoints

[pigmej]

being able to get the gracePerdiod via API would be handy in some cases.

The open question is whether it should be in the PoetInfo endpoint or more like the "node" endpoint, as the setting is PER node, not PER poet.

[tjb-altf4]

the setting is PER node, not PER poet.

is this still the case if running multiple smeshing-services connecting to a single node?

if not it would be nice to still be able to retrieve the inherited setting (from the node) via the smeshing-service, so one does not have to think about which node to query for gracePeriod in case of more complex configurations