Add UUID auto-id

[davidmezzetti]

#502 added the ability to generate auto-id's when not otherwise provided at index time. This change will enable generating UUIDs via the autoid parameter. The logic should support any of the UUID functions available to generate random and deterministic UUIDs.

For example:

from txtai.embeddings import Embeddings

embeddings = Embeddings()
embeddings.index(["test"])

Currently creates an entry as (0, "test", None).

and with this change:

from txtai.embeddings import Embeddings

embeddings = Embeddings(autoid="uuid4")
embeddings.index(["test"])

Will instead create this entry as ("aa108dc5-442c-458a-8454-8c55f40af25d", "test", None).

The default method when not specified will continue to be a numeric sequence.