[Searchable Snapshot] Create prototype for read only engine backed by remote store

[andrross]

A snapshot-backed remote index will need an implementation of a read only implementation of an Engine. This task is to create a basic design to build a prototype of such an engine implementation. This task should be done in collaboration with the segment replication work in this read (#2200) because there may be some overlap (a replica in segment replication is in some sense read-only because it is not doing any indexing work and instead just refreshing segments from the primary node doing the indexing).

• Define APIs needed for Engine #2925

[andrross]

Closing this as we did develop a rough prototype on a user fork to help answer some of the unknowns here: https://github.com/andrross/OpenSearch/tree/remote-snapshot-experiment-2

The overall project is tracked in the meta issue (#2919)