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