header/store: GetRange to return sub range

As celestiaorg/celestia-node#1807 got merged, the Exchange Server now implements the ability to return partial responses to clients. E.g. server may not have everything in the request but can give what it has. This is currently implemented in Server, while the Store can solve this case as well. In the end we will have lesser code and more powerful/universal API of the Store

Start by reading the Store and Server implementations, especially the existing GetRange behavior and the partial-response handling described in celestiaorg/celestia-node#1807. Determine the expected Store API semantics for requests that cannot be fully satisfied, then confirm that Store can return the available sub-range with behavior matching Server.

Written by the indexing model from the issue text.