[Remote Store] Support nested index level settings for remote store

[sachinpkale]

Describe the solution you'd like
Current setting index.remote_store removes the flexibility of adding few sub-settings related to remote store for the same index. In the future, we can have requirements like supporting only remote trans-log, separate remote store for given index. If we use index.remote_store.enabled, all the remote_store settings can be included under index.remote_store.