[Remote Store] Prefer remote replicas for primary promotion during migration

[ltaragi]

Is your feature request related to a problem? Please describe

• During remote store migration (MIXED compatibility mode and REMOTE_STORE migration direction), we aim to continuously move forward.
• To ensure this, in case of a primary shard failover:
  • If the primary and one or more replica copies had already migrated to remote nodes, then the new primary must be chosen randomly from the migrated replicas
  • If no migrated replica is found, fall back upon the existing mechanism

Describe the solution you'd like

Inside unassignPrimaryAndPromoteActiveReplicaIfExists in RoutingNodes.java:

• metadata can be used to identify migration conditions
• Using the failedShard's id, we can look for migrated replica copies and implement the above preference

Related component

Storage:Remote

Describe alternatives you've considered

No response

Additional context

No response