Add extension for 'do not explore cids'

[willscott]

Currently, graphsync supports an extension for doNotSendCIDs, which provides a list of CIDs that should not be sent during a transfer.

It may be the case that the requester wants to receive a dag, but wants to stop graph retrieval whenever a CID is reached. For instance, consider the use case of synchronizing a chain, but only back to the point of previous synchronization.

This could either be done as an extension to graphsync, as proposed here, or incorporated into the IPLD selector syntax.

Adding an extension here may remain useful as a way to more efficiently resume transfers with the ability to collapse the described 'cids i already have' to just roots of subtrees that have been received previously. The graphsync extension path also is likely a faster path for prototyping this behavior than the selector alternative.

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[willscott]

After considering this more, i think the right way to do this is in selectors.
Extended that within the context of existing spec definition in
ipld/go-ipld-prime#214

[willscott]

merged at the ipld-prime layer