Internal: split internal fetch request used within scroll and search

Similar to #7856 but relates to the fetch shard level requests. We currently use the same internal request when we need to fetch within search and scroll. The two original requests though diverged after #6933 as `SearchRequest` implements `IndicesRequest` while `SearchScrollRequest` doesn't. That said, with #7319 we made `FetchSearchRequest` implement `IndicesRequest` by making it hold the original indices taken from the original request, which are null if the fetch was originated by a search scroll, and that is why original indices are optional there.

This commit introduces a separate fetch request and transport action for scroll, which doesn't hold original indices. The new action is only used against nodes that expose it, the previous action name will be used for nodes older than 1.4.0.Beta1.

As a result, in 1.4 we have a new `indices:data/read/search[phase/fetch/id/scroll]` action that is equivalent to the previous `indices:data/read/search[phase/fetch/id]` whose request implements now IndicesRequest and holds the original indices coming from the original request. The original indices in the latter request can only be null during a rolling upgrade (already existing version checks make sure that serialization is bw compatible), when some nodes are still < 1.4.
Can we add docs here and explain why we need it in addition to `ShardFetchRequest`?