Internal: split shard search request into a local and a transport variant

In some cases a shard search request gets created on a node to be only used there and never sent over the transport. This commit clarifies that and creates a new base class called `ShardSearchLocalRequest` that can and will be only used locally. `ShardSearchTransportRequest` on the other hand delegates to the local version but extends `TransportRequest` and is `Streamable`, which means that it is supposed to be sent over the transport.

This way we can make the `OriginalIndices` only required (and mandatory now) in the transport variant.

Took the chance to remove an unused InternalScrollSearchRequest constructor and an empty else branch in `TransportSearchScrollQueryAndFetchAction`.

Closes #7855