[protobuf] can't parse `reserved` field

This is a Protoparser problem that it failed to parse `reserved` field.

Reserved field is explained in the official document.
https://developers.google.com/protocol-buffers/docs/proto#reserved

```
va.lang.IllegalStateException: Syntax error in Unnamed-protobuf-schema at 389:11: unexpected label: reserved

at com.squareup.protoparser.ProtoParser.unexpected(ProtoParser.java:948)
at com.squareup.protoparser.ProtoParser.readDeclaration(ProtoParser.java:197)
at com.squareup.protoparser.ProtoParser.readMessage(ProtoParser.java:219)
at com.squareup.protoparser.ProtoParser.readDeclaration(ProtoParser.java:153)
at com.squareup.protoparser.ProtoParser.readProtoFile(ProtoParser.java:92)
at com.squareup.protoparser.ProtoParser.parse(ProtoParser.java:61)
at com.fasterxml.jackson.dataformat.protobuf.schema.ProtobufSchemaLoader._loadNative(ProtobufSchemaLoader.java:167)
at com.fasterxml.jackson.dataformat.protobuf.schema.ProtobufSchemaLoader._loadNative(ProtobufSchemaLoader.java:161)
at com.fasterxml.jackson.dataformat.protobuf.schema.ProtobufSchemaLoader.loadNative(ProtobufSchemaLoader.java:135)
at com.fasterxml.jackson.dataformat.protobuf.schema.ProtobufSchemaLoader.load(ProtobufSchemaLoader.java:80)
```