RFC 629: Wire up log stashers for `lsif_indexes`

Created by: efritz

Implements a portion of RFC 629: Cold-storing older worker execution logs.

Instantiate the log stasher created in #33025 and target the lsif_indexes table. This will ultimately add tasks to the worker service. As a first pass, the chosen sink should be the null sink introduced in #33026. The age of affected records should be configurable via an environment variable. Running the stasher should be guarded by an environment variable that is initially off by default.