lncfg/interface: define Validator iface + variadic Validate

This commit introduces the Validator interface, which
is intended to be implemented by any sub configs. It
specifies a Validate() error method that should fail
if a sub configuration contains any invalid or insane
parameters.

In addition, a package-level Validate method can be
used to check a variadic number of sub configs
implementing the Validator interface. This allows the
primary config struct to be extended via targeted
and/or specialized sub configs, and validate all of
them in sequence without bloating the main package
with the actual validation logic.

lncfg/workers.go

@@ -47,3 +47,6 @@ func (w *Workers) Validate() error {
 
 	return nil
 }
+
+// Compile-time constraint to ensure Workers implements the Validator interface.
+var _ Validator = (*Workers)(nil)