feature request: support extensible data fields

I would like to add support for [extensible data](https://specs.ipfs.tech/ipns/ipns-record/#extensible-data-dag-cbor) to this implementation of the IPNS record.

This could be done by:

1. adding an optional `extensibleData` field to `CreateOptions`.
2. adding an optional parameter `extensibleData` to [`createCborData`](https://github.com/ipfs/js-ipns/blob/e612553dab45bf102ef1f5b239fc4e13c0f96f3f/src/utils.ts#L222).
3. inside of createCborData, merge extensibleData the `data` object.
4. add tests and documentation

It's not necessary to change types or add generics to the record type. Users of this feature would simply have to decode `IPNSRecord.data` to read any extensible fields.