[Remote Migration] Support CCR while migrating to/from remote store

[gbbafna]

Is your feature request related to a problem?

We will be supporting migration of docrep clusters to/from remote clusters. During the migration , we need to support ongoing and new cross cluster replications .

We need to think about how it is going to pan out for both source cluster and target cluster undergoing these migrations.

What solution would you like?

For Source Cluster undergoing migration

For remote store backed cluster, ccr relies on lucene segments instead of translogs. We need to do same when the cluster is in mixed mode.

A cluster in mixed mode can have shards in remote store mode . An index can have some shards in remote mode and some in doc rep mode during mode.

For Target Cluster undergoing migration

Needs more thought.

Do you have any additional context?

Add any other context or screenshots about the feature request here.