Restrict async translog durability mode for remote indexes

[ashking94]

Is your feature request related to a problem? Please describe.
Remote enabled indexes provide user capability to have request level durability when the translog durability mode is request. Currently, as status quo, we provide user the capability to create both request and async durability mode. The user can also switch between these 2 modes. Since remote store enables user to achieve strong durability with request level durability, it poses risk to lose that durability if the durability mode is switched from request to async. With this, the proposal is to have a cluster setting which if enabled restricts creation or updation of an index with translog durability mode "async".

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