Prepare subspace-farmer-components to retrieve pieces in batches

[nazar-pc]

This introduces PieceGetter::get_pieces() (with dummy implementation that just calls PieceGetter::get_piece() internally) and prepares subspace-farmer-components to use it.

This prepares lower-level part of the farmer implementation, next steps include properly implementing PieceGetter::get_pieces() in FarmerPieceGetter and ClusterPieceGetter to fully utilize new capabilities and then switching piece cache sync to PieceGetter::get_pieces().

Eventually we'll make similar changes in subspace-service and upcoming data retrieval application.

I'm also considering optimizing PieceGetter::get_piece() to take advantage of already connected peers if there remains a use case for it after all this, though I'll need to check how much sense there is since we can just call with a single piece index, though I realize we'll have a bunch of extra bookkeeping there that might be too heavy for the use case.

Note that in recover_missing_piece implementation is not of the highest possible efficiency (it waits for one batch to complete before starting to download more pieces), but that is a conscious decision to simplify implementation given that requiring reconstruction is an exceptional situation to begin with and should be a very cold code path overall, it really shouldn't happen on any real network in practice.

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