[Fix] Allow custom actions with flags in config validation

[Fix] Allow custom actions with flags in config validation

The config validation was warning about unknown actions even when they
were valid custom actions defined with flags like 'no_threshold'.

Custom actions (e.g., malware, virus, hard-reject) are properly supported
by the C code but the Lua validation didn't recognize them, causing
spurious warnings.

Fix by checking if an unknown action is defined as an object with flags
before emitting the warning.