Make zoekt indexer testing in the monolith easier

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Background

This is related to !167455 (comment 2153248236)

Currently, we can't properly test Zoekt task-based indexing in CI. We're using the legacy API to trigger indexing.

Proposal

I believe that we should add a new endpoint POST /indexer/tasks so that Rails could push tasks (only for CI/dev). We can use exactly the same formatted tasks we send via the tasks API.

Another benefit is that we can deprecate the old (legacy) endpoints.