autoindex job is created on-demand

Created by: michaellzc

as of today, auto index job is run on a fixed schedule https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/codeintel/autoindexing/background/scheduler/config.go?L27

it would be great to be able to run this on demand so we will always have precise code intel for all codes

learn more: https://sourcegraph.slack.com/archives/CHXHX7XAS/p1657033725832799