codeintel: Add blocklist for autoindexing

[efritz]

Fixes #51430. This PR adds the ability to mark repos as blocked for index scheduling and for inference. At the point where these repos are about to perform such an action, it is logged and no-op'd.

This should be exposed via GraphQL eventually but isn't a priority for this PR.

Test plan

Updated unit tests, tested locally.

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@Strum355

enterprise/internal/codeintel/autoindexing/internal/background/dependencies/mocks_test.go enterprise/internal/codeintel/autoindexing/internal/jobselector/job_selector.go enterprise/internal/codeintel/autoindexing/internal/store/config_repo.go enterprise/internal/codeintel/autoindexing/internal/store/config_repo_test.go enterprise/internal/codeintel/autoindexing/internal/store/observability.go enterprise/internal/codeintel/autoindexing/internal/store/store.go enterprise/internal/codeintel/autoindexing/mocks_test.go enterprise/internal/codeintel/autoindexing/service.go enterprise/internal/codeintel/autoindexing/service_test.go