Add force flag to zoekt

What does this MR do and why?

This MR adds an optional force option to Zoekt::IndexerWorker. It's backwards compatible with older indexers if we do not set this flag. The flag was added it to the indexer's API in Add force option to the index API (gitlab-zoekt-indexer!84 - merged)

The idea behind it so that we can perform reindexing CRs like gitlab-com/gl-infra/production#17105 (closed) with a lot less downtime for the customers.

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