Add a cron worker to detect and resize shards using zero downtime reindexing

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Background

All index shard resizing is manually monitored and fixed using change requests. This doesn't scale and the team needs to find an automated way to do this.

Proposal

• take the existing migration helper code and turn it into a service
• call the service from the migration helper
• add a new cron worker (to be run monthly) that calls the service
• the worker should be added in EE
• the worker should only run if advanced search indexing is enabled (feature is turned on)
• the worker should check the existing index settings in the database for shard size per index