Feature request: listpeerchannels should allow querying by SCID

[whitslack]

While managing my node, I frequently want to see the details of a particular channel whose SCID I readily know but whose peer node ID I do not have at hand. It would be nice if listpeerchannels could accept a SCID as its id argument as an alternative to a node ID.

And yes, I know I can pass a SCID to listchannels to find the node ID of the peer, although that seems only to work for published channels that gossipd knows about. (I have failed to find my own private channels using that command.) Alternatively, I know I can also dump all of my channels by calling listpeerchannels with no id argument and then filtering the results manually, but it would be nice if I didn't have to jump through all the extra hoops. Asking listpeerchannels for a particular channel by SCID seems natural.