support header specifying additional root cid

[willscott]

there may be a 'higher' root cid than the one requested that the client suggests will be more useful for finding peers only advertising their root CIDs.

This CID may be communicated in a header - lassie should do a content routing lookup also for this cid when present.

• confirm name of the header
• pipe the new header value into content routing sub-system

[hannahhoward]

Notes:

We can probably kick off a second HTTP request in the content routing subsystem to IPNI and just combine the two streams and use the CandidateBuffer to combine the streams

[rvagg]

Assuming this is coming in through the Gateway->Saturn path, how is the second CID originating from the source requestor in that flow? Is it something like an upstream component having part of the DAG but wanting to fill in holes?