Added code to add instances into the subclustered placement

[RiyaTyagi]

What this PR does / why we need it:

Following changes have been added with this pull request:

• The support for AddReplica during the subclustered placement has been removed. Once created the placement cannot have more replicas added later.
• Methods for marking all shards available or shards of an instance available has been implemented reusing the code from sharded placement algorithm.
• A new method newubclusteredAddInstanceHelper has been added which checks if the instance already exists in the placement or not. If the instance doesn't exist it assign a subcluster ID to it and then called the newubclusteredHelper method. If the instance is already present in the subcluster we check if all the shards of the instance are in leaving state otherwise an error is thrown.
• The addInstance method has same implementation as present in sharded algorithm with a slight change in assignTargetLoad method implementation.
• In the assignTargetLoad method we first try to assign shards from the instances belonging to subclusters other than the current adding instance subcluster.
• Once we exhaust all the options from other subcluster's instances we try to move shards from the addingInstance's subcluster's existing instances.
• Utility methods to get subcluster's instances and remove a particular subcluster's instances have also been added.

[RiyaTyagi]

This change is part of the following stack:

• Changed the interface of placement objects to include subcluster information #4355
  • Added the placeholder interface methods for the subclustered algorithm #4356
    • Added the subclustered helper file with placemnt helper methods placeholders #4357
      • Added method to validate the weights of instances and scan current load #4358
        • Added method to build target shard count for subcluster and target load per instance #4359
          • Added code for assigning subcluster Ids and adding validations in initial placement #4360
            • Added code for initial placement #4361
              • Added code for validating subclustered placement #4362
                • Added code to add instances into the subclustered placement #4363 ◀
                  • Added code to greedily choose the shards to minimize skew in existing subcluster #4364
                  • Added code to move shards from the existing instances to new instances maintaining subcluster property #4365
                  • Added code to remove instances/subcluster from the placement #4366
                  • Added code to replace instances in subclustered placement #4367
                  • Added code to balance shards #4368

_{Change managed by git-spice.}