[Enhancement] Enhance GatewayAllocator to handle a batch of shards fetching

[amkhar]

Describe the bug
Original Issue opened : #5098 . This issue is a sub task for the overall project defined #8098.

Description
As part of node drop and joins, GatewayAllocator works on assigning single shard in one call and maintains a map where key is shardId and value is a shard level fetcher (shard level fetcher is responsible for fetching the metadata over transport for single shard).

OpenSearch/server/src/main/java/org/opensearch/gateway/GatewayAllocator.java

Lines 172 to 185 in 96630f0

	protected static void innerAllocatedUnassigned(
	RoutingAllocation allocation,
	PrimaryShardAllocator primaryShardAllocator,
	ReplicaShardAllocator replicaShardAllocator,
	ShardRouting shardRouting,
	ExistingShardsAllocator.UnassignedAllocationHandler unassignedAllocationHandler
	) {
	assert shardRouting.unassigned();
	if (shardRouting.primary()) {
	primaryShardAllocator.allocateUnassigned(shardRouting, allocation, unassignedAllocationHandler);
	} else {
	replicaShardAllocator.allocateUnassigned(shardRouting, allocation, unassignedAllocationHandler);
	}
	}

OpenSearch/server/src/main/java/org/opensearch/gateway/GatewayAllocator.java

Lines 82 to 87 in 96630f0

	private final ConcurrentMap<
	ShardId,
	AsyncShardFetch<TransportNodesListGatewayStartedShards.NodeGatewayStartedShards>> asyncFetchStarted = ConcurrentCollections
	.newConcurrentMap();
	private final ConcurrentMap<ShardId, AsyncShardFetch<TransportNodesListShardStoreMetadata.NodeStoreFilesMetadata>> asyncFetchStore =
	ConcurrentCollections.newConcurrentMap();

GatewayAllocator now needs to take care of bookkeeping of multiple batches, where single batch can have multiple shards. One key constraint is that one batch will be fixed for a set of shards and we won't add any new shards in a batch after batch is created.

Functionalities required

• Keep fetchers at batch level also in a separate map
• Create batches for assigning all unassigned shards
• Keep necessary data structures so finding the batch for a shard or find shards for a batch is easy
• Cleanup the newly created maps accordingly when the whole batch assignment is finished

Additional context
Please go through the discussion #5098 to understand the overall enhancement approach and check #8098 to see the sub tasks of overall project.