metadata in link json

When managing collections of neuroglancer links (as JSON documents), it would be convenient if I could sneak in a bit of metadata for myself. I wouldn't want the metadata to be actually used by neuroglancer in any way, but I would want it retained in the JSON state (and URL).

For example:

```json
{
"metadata": {
"link-created-by": "bergs",
"link-creation-date": "2025-02-10"
},
"title": "my dataset",
"dimensions": [...],
"layers" [...],
"layout": "4panel"
}
```

(I have additional metadata fields in mind that I won't bother explaining here.)

Would it be possible to permit a schemaless user-provided metadata field in the neuroglancer JSON state? (I don't have an opinion on the name of the key... `metadata` or `user-defined-metadata` or something like that seems fine.)

cc: @olbris