refactor(share): GetShare -> GetSamples

[Wondertan]

Samples were requested individually rather than grouped. This was causing issues for Bitswap in the mode where each request is a distinct session, so the solution is to start grouping those as we originally planned.

Mainly removes GetShare and introduces:

GetSamples(ctx context.Context, header *header.ExtendedHeader, indices []SampleIndex) ([]Sample, error)

And introduces SampleIndex with helper funcs.

The current state of the PR unblocks me and allows me to iterate further on Bitswap issues, while the remaining TODOs still need to be finished.

TODOs:

• eds.Accessor should take the new shwap.SampleIndex as param in Sample method
• !Add GetSamples to the ShareModule.
  • Need to decide which release we want to put this breaking change in
• Godoc new things
• Fix remaining tests
• Rebase
• Decide on whether SampleIndex should be an index or a tuple.

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