[Searchable Snapshot] Implement an API for creating an index backed by a remote snapshot

[andrross]

Implement an API for creating an index backed by a remote snapshot (starting with a naive storage layer to prove out the end-to-end functionality), gated by a feature flag.

A rough sketch of what this should look like exists here: https://github.com/andrross/OpenSearch/pull/102/

There are a few known issues that need to be solved:

• IndexShard::openEngineAndRecoverFromTranslog() doesn't work properly because there should be no translog for this type of shard. The above branch has a line commented out so a proper solution needs to be developed here.
• BlobStoreRepository::assertSnapshotOrGenericThread has been commented out
• IndexShard::createShard() should use the directory factory to create the appropriate directory, as opposed to the hacky branching logic that currently exists
• The new index settings should be scoped to Property.InternalIndex and validated that this approach will work.