Selectors Are Part Of Retrieval Deal Proposal

[hannahhoward]

As a creator of a retrieval deal, I want to be able to add a selector to a retrieval deal proposal, so I can request data that is not an entire payload tree

Scope:

• bit tricky with selector because it's an ipld.Node interface. CBOR gen structs cannot include interfaces
• Add selector to DealProposal as a *cbg.Deferred (see go-hamt-ipld for example)
• Add a NewParamsV1 method (similar to NewParamsV0) which takes a Selector ipld.Node, uses go-ipld-prime to serialize it to a byte array (see https://github.com/ipfs/go-graphsync/blob/master/ipldutil/ipldutil.go#L50 for example)
• Take the byte array returned and set cbg.Deferred Raw value based on that (again, see https://github.com/ipfs/go-hamt-ipld/blob/master/hamt.go#L236) for example

Out: actually doing anything with selector