[Remote Store] Mixed mode support for remote store migration

[gbbafna]

Is your feature request related to a problem? Please describe

To enable migration of document replication clusters to remote store backed clusters, we will need to allow mixed mode clusters .

Describe the solution you'd like

The behavior is going to be as followed

1. STRICT Mode - Only one type of node (remote or docrep node) is allowed to join cluster.
2. MIXED Mode - Any type of node (remote or docrep node) is allowed to join cluster. We will also need another DIRECTION setting to dictate which nodes the new index is going to land . MIXED Mode should only be allowed in an experimental flag till we add complete support for same.

Related component

Storage:Remote

Describe alternatives you've considered

No response

Additional context

No response