Support automatically updating index when content is published

[tomusher]

At the moment the only way to update the indexes is to run the update_vector_indexes management command. This gives developers control over when embedding API costs are incurred.

It would be nice if there was an option to add/update items in the index when they are published/saved.

Some considerations:

• As this could get expensive on edit-heavy sites, it might make sense to have this feature disabled by default
• There may be cause to only auto-update certain models rather than every registered model/index.

[tm-kn]

Should we have a way to run this on Celery given that this is an external API call and advise not to use it in the request/response cycle?

[Morsey187]

I've opened up a draft PR here #30, however theres a few points that need resolving before this can be completed.

Issue: Signals are part of the request cycle and updating indexes can be time consuming, we should add support for a task queue and consider whether we'd want to allow using these signals without one at all.
Issue: Currently requires rebuilding the whole index, instead of updating, we'd need to figure out:
Which indexes a model is in (so we can update the right indexes)
A way to remove documents from an index that match a given set of metadata (the object id and content type ID in this case)
An easier way to generate embeddings on a per-document level, instead of at the rebuild index stage

[tm-kn]

For reference, @mgax implemented this as part of his project, but we'd likely have the same issues as above. I don't think @mgax's solution rebuilds the whole index.

Maybe we need to make it a bit less automatic or perfect so we can merge this in some form?