[Feature Request] Unify remote store related download operations under a single DownloadManager

[kotwanikunal]

Is your feature request related to a problem? Please describe

Parallel/MultiStream downloads need to accommodate for a unified DownloadManager to perform downloads from the repository for remote indices (remote store/remote index/searchable snapshot)

Currently, download managers are fragmented and perform the same download operation from the repository using duplicated code with minor changes.

Describe the solution you'd like

A unified DownloadManager will help with the remote index (remote store/remote index/searchable snapshot) based requests.

• This task aims at unifying the RemoteStore related download operations under a single DownloadManager and wiring it up across the two use cases

Related component

Search:Remote Search

Describe alternatives you've considered

No response

Additional context

No response