Add slog for index repository start/end

What does this MR do and why?

During a pair session today, Dmitry and I paired on adding a quick log entry when starting and finishing the index repository method.

How to set up and validate locally

1. stop gdk version of zoekt indexer gdk stop gitlab-zoekt-indexer-development
2. make gdk
3. force index a repository in rails console: Zoekt::IndexerWorker.new.perform(2, { force: true })
4. monitor logs

time=2024-03-27T15:36:49.811-04:00 level=INFO msg="IndexRepository start" project_id=2
time=2024-03-27T15:36:50.279-04:00 level=INFO msg="finished /Users/terrichu/projects/gitlab-development-kit/zoekt-data/development/index/2_v16.00000.zoekt: 536512 index bytes (overhead 4.1)"
time=2024-03-27T15:36:50.280-04:00 level=INFO msg="IndexRepository end" project_id=2