Minor fix to include signed ints in Protobuf transform (#13) (#2286)

Author: rayokota

src/Confluent.SchemaRegistry.Serdes.Protobuf/ProtobufUtils.cs

@@ -244,11 +244,13 @@ private static RuleContext.Type GetType(FieldDescriptor field)
                 case FieldType.Bytes:
                     return RuleContext.Type.Bytes;
                 case FieldType.Int32:
+                case FieldType.SInt32:
                 case FieldType.UInt32:
                 case FieldType.Fixed32:
                 case FieldType.SFixed32:
                     return RuleContext.Type.Int;
                 case FieldType.Int64:
+                case FieldType.SInt64:
                 case FieldType.UInt64:
                 case FieldType.Fixed64:
                 case FieldType.SFixed64: