Add wildcard for attributes

For data-centric layers (e.g. dynamically updated layers containing lots of additional info for features) it will be useful to allow a wildcard definition for the layer fields:

```
"fields": {
"*": "All attributes"
}
```

This let's an application know that there are various attributes available, without being limited to a predefined set.

Start by locating the TileJSON specification section that defines layer fields and review how field definitions are represented. Add support for the proposed "*" wildcard with its descriptive value, then verify that the specification clearly permits dynamic attributes without requiring a predefined set.

Written by the indexing model from the issue text.