autoindex config is more accessible from site-config

Created by: michaellzc

right now, all autoindex scheduling related configs are coming from env var, and they're annoying to change on the fly https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/codeintel/autoindexing/background/scheduler/config.go?L27

it would be great to include them in site-config