Add map callback to bulk helper

[Mpdreamz]

Right now you pass a stream of TDocument to client.helpers.bulk.

If you need to take contextual decisions in the onDocument callback TDocument might not be enough. It would be helpful if we can pass a stream of wrapped documents and unwrap the actual document to serialize using a map handler to client.helpers.bulk()

[delvedor]

Closed in #1606.

[lennartnederstigt]

@delvedor How's this issue actually addressed by #1606? Doesn't seem to be the case, unless I'm missing something. I'd really like this functionality as well.

[Mpdreamz]

++ #1606 fixes the generic being propagated properly but does not expose a way to map() the passed TDocument type to the type we actually want to write in the bulk update/create/index.

[JoshMock]

This has been improved in #1732, which will be shipped in 8.8.2 and 8.9.0 next week. Docs for how to use this improvement are in #1951.