[BE] Step 2: Support multiple indices to Elasticsearch Admin options

⚠ This issue is not released yet, all the involved MRs are merged into !17230 (closed) and the remainder of this work is being tracked in &1769.

---

This is a placeholder for backend related work for #13534 (closed)

The work should continue on from

1. !15736 (closed)
2. !15275 (merged)

Needed changes

• Controller actions for indices
  • index/new/edit
  • remove: if currently active, make other index active
  • activate
  • reindex
  • pause/resume indexing
• ee/lib/gem_extensions/elasticsearch/model/client.rb
  • This class manage to expire elasticsearch client object, whenever ApplicationSetting changes
  • Now it has to handle Index record changes too.
  • Cache clients per index, check changed settings per index
• Elastic::MultiVersionClassProxy
  • We need to have it load the indices from database
  • Different for read operations and write operations.
• Store IndexStatus per index
  • Add elasticsearch_index_id column
  • Figure out how we can track this from the Rails side
• Update/remove rake tasks