Active Context: Add AdHocIndexingWorker

Description

This adds a new AdHocIndexingWorker for Active Context, enabling on-demand indexing of individual projects based on GitLab Duo usage. Unlike ProcessPendingEnabledNamespaceEventWorker, which indexes the entire set of eligible projects, this worker allows selective indexing. To support this, we need to create EnabledNamespace records with state=ready, while preserving the ability to manually index a namespace by setting its state to pending.

Reference: https://docs.gitlab.com/development/sidekiq/

Related to #577330