Add API to pass custom data to observers

From the discussion in #1870

What do you think about adding custom data to a query/batch that is passed to observers? We use a context.WithValue() to pass query names to observer, but this approach can lead to errors because engineers can use the same context for different queries. Would be great to have an approach when data is assigned for individual queries.

Start by reading the discussion in #1870 and tracing the query, batch, and observer APIs mentioned in this issue. The work is done when custom data can be assigned to an individual query or batch and observers receive it without relying on a shared context.WithValue().

Written by the indexing model from the issue text.