Add rake task to resize an index's 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

• Add a new rake task (see gitlab:elastic rake tasks and Search::Elastic::RakeTaskExecutorService) to resize an index's shards using zero downtime reindexing. The task should:
  • should calculate the new shard size required
  • update the shard setting in the database
  • run zero downtime reindexing for that specific index
• Add specs for the task
• Add docs about the task to the advanced search admin docs