[FEATURE] Allow configuring concurrent segment search per-cluster and per-index

[scottlaird-figma]

Is your feature request related to a problem?

We would like to be able to turn concurrent segment search on and off via Terraform. Without this, we need to manually enable or disable it via Opensearch's dashboard, which leads to inconsistency.

What solution would you like?

Add a search_concurrent_segment_search_enabled setting and possibly a search_concurrent_segment_search_mode (for OS 2.17 only) setting to both opensearch_index and opensearch_cluster_settings.

What alternatives have you considered?

We can set this manually, but we have quite a few clusters and inevitably we lose track of where it's been enabled and disabled. Using Terraform would bring this in line with other config settings that we manage.