Add a `uuid` field type

Is your feature request related to a problem? Please describe. I'm using uuids as ids in my application, but tantivy doesn't support uuid fields!

Describe the solution you'd like Add uuids as a field type.

[Optional] describe alternatives you've considered

• add_bytes: works, but is a bit unergonomic and might take up more space than if there was just a uuid(?) (unsure if this is true, haven't looked too deeply into tantivy's internals)
• two add_u64s: even more unergonomic