Expose the NAR calculation to a separate `CalculateNAR` method, which
responds with the NAR size and sha256 hash.
Contrary to what cl/7618 and cl/7620 initially did, don't add different
other request types.
In the CalculateNARResponse message, there's now some duplication in the
(optional) `narinfo` field of a PathInfo, but I'm not entirely sure if
we want to drop the fields from there yet.
Change-Id: Id797c56e17efedac115fbd43de9dfde9fa1db140
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7663
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
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