[18.10] Zero downtime reindexing make setting async-durability optional

What does this MR do and why?

Backport of !226356 (merged) to 18-10-stable-ee.

Fixes a bug where zero-downtime reindexing would get stuck indefinitely in indexing_paused state when the connected search backend (AWS OpenSearch Service, or self-managed OpenSearch with cluster.remote_store.index.restrict.async-durability=true) rejects the translog: { durability: async } setting with a BadRequest error.

This backport cherry-picks 7dd9f8f3aaf908898b88a8b31b7ffc57c855349e cleanly with no conflicts.

MR acceptance checklist

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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