validate-config sub command for casper-node

[devendran-m]

trafficstars

This would start up and exit with 0 if no issue and probably panic like normal if deserialization error occurs. Would be very helpful in proving that existing config would not crash after upgrade.

[EdHastingsCasperAssociation]

We could add this, it does not require a breaking change, it is relatively low effort. I would estimate as 3pts, with testing to be done downstream by SRE.