Add ResolvePredefinedConstraints method to DefaultResolver (#151)

This method is not really needed by protovalidate itself, but it is
really convenient for us to implement and really inconvenient for
consumers to implement, especially if they want to mimic the reparsing
logic of protovalidate-go.

Fixes #147.

resolver/resolver.go

@@ -40,3 +40,11 @@ func (r DefaultResolver) ResolveOneofConstraints(desc protoreflect.OneofDescript
 func (r DefaultResolver) ResolveFieldConstraints(desc protoreflect.FieldDescriptor) *validate.FieldConstraints {
 	return extensions.Resolve[*validate.FieldConstraints](desc.Options(), validate.E_Field)
 }
+
+// ResolvePredefinedConstraints returns the PredefinedConstraints option set for
+// the FieldDescriptor. Note that this value is only meaningful if it is set on
+// a field or extension of a field rule message. This method is provided for
+// convenience.
+func (r DefaultResolver) ResolvePredefinedConstraints(desc protoreflect.FieldDescriptor) *validate.PredefinedConstraints {
+	return extensions.Resolve[*validate.PredefinedConstraints](desc.Options(), validate.E_Predefined)
+}