Private-use messages

Nobody has claimed this yet.

Creators and users of vector tiles should be able to include private messages within tiles, layers, and features, by mutual agreement. Other software that doesn't know how to interpret these messages should pass them through as opaque blobs to the extent that is possible.

Something like:

```
message private {
required string = 1;
extensions 20 to max;
}
```

and then in each of `tile`, `layer`, and `feature`,
```
repeated private = 6;
```

(or some other number).

One of the options for `value` could also be an object of this type if there is a need for opaque feature attributes.

The `string` in the `private` message would be the URL for the specification for the private message type. There is no need for software to actually download a file from the URL; it just serves to distinguish one type of private message from another.

No file or test entry point is named. Review the vector-tile specification and the existing tile, layer, and feature definitions, then determine whether the proposed private-message extension is accepted and how opaque pass-through should be specified; done would be an agreed schema and documented behavior.

Written by the indexing model from the issue text.