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/interface.go

@@ -0,0 +1,21 @@
+package lncfg
+
+// Validator is a generic interface for validating sub configurations.
+type Validator interface {
+	// Validate returns an error if a particular configuration is invalid or
+	// insane.
+	Validate() error
+}
+
+// Validate accepts a variadic list of Validators and checks that each one
+// passes its Validate method. An error is returned from the first Validator
+// that fails.
+func Validate(validators ...Validator) error {
+	for _, validator := range validators {
+		if err := validator.Validate(); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}

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)