[Remote Store] Add support to restore only unassigned shards

[sachinpkale]

Is your feature request related to a problem? Please describe.

• Currently, remote store restore API restores all the shards of a given index irrespective of the shard state.
• This approach is not optimized for scenarios where only few of the shards are unassigned. Restore will still download data for all the shards in order to restore them.
• We also need to close the index before calling remote store restore API. This impacts the index availability during the restore.

Describe the solution you'd like

• Ideally, we should be able to restore only those shards that are unassigned and that too without closing the index.
• In restore API execution, while marking shards to be restored, we will filter shards based on existing ShardRoutingState.
• We pick only shards that are in UNASSIGNED state as shards to be restored.
• User do not need to provide shard info in the API request. API signature would remain same which will accept index/indices to be restored. API implementation internally picks up only unassigned shards to restore.
• We will also provide a query parameter restore_all_shards which, as the name suggests, will restore all the shards of the given index. As some of the shards can be active, to avoid an inconsistent state, when restore_all_shards=true is passed, index needs to be in closed state.