Enable selector as an optional parameter for constructing an input for the dealbot

[jnthnvctr]

For Filecoin Discover, we'd like to be able to retrieve sub-portions of our deals (to be able to confirm Miners are honoring our requests for fast/free retrieval without requiring them to ship us back 1GiB per test).

In order to do this, we'd need the dealbot to accept a selector as an optional input to each json object for triggering the retrieval request.

cc @ribasushi

[dkkapur]

@jnthnvctr @ribasushi is this still required for Discover? Trying to triage asks on dealbot to be completed in Q1 2022 onwards, not sure what the status of this ask is.

[ribasushi]

@dkkapur still needed