share: GetShares method for Getter interface

[Wondertan]

We only have the GetShare method, which gets us a single share/sample. It worked well before, but with shwap sampling protocol, we need a method that gets multiple shares within one call to allow batching requests.

GetShares(ctx context.Context, hdr *header.ExtendedHeader, shrIdxs ...int) ([]share.Share, error) 

The method can also satisfy singular requests, so we would also deprecate GetShare

[vgonkivs]

We had something similar but with range. I think we can combine somehow these two issues:
#2304

[Wondertan]

I dont see how we can combine range requests and single share requests in a single method ergonomically. Please show if you have an idea

[vgonkivs]

instead of having shrIdxs ...int it could be some kind of a filter struct that determines the behavior.

[vgonkivs]

type filter struct {
idx []uint
start uint
end uint
}

func(f *filter) Next() uint {}

[renaynay]

Resolved by #3891