Add streaming support for 'query' calls

[tomusher]

When building a 'chat'-like interface, it can be a nicer experience for responses to stream in rather than waiting for it all to be ready.

Implement a way to stream these responses, e.g. using server-sent events.

This should be an optional feature as it will likely require additional infrastructure.

[tomusher]

In progress on #28

[Morsey187]

In review, theres a few issues related to ASGI that still needs to be resloved for the related intranet work being used to test this, however, I've set aside some time each day address this.

• https://github.com/wagtail/wagtail-vector-index/pull/28

[tomusher]

Reviving this as #86