lightningd: add short_channel_id option to listpeerchannels.

[rustyrussell]

Requested-by: @whitslack
Closes: #8233
Changelog-Added: JSON-RPC: listpeerchannels now has a short_channel_id parameter for just listing a specific channel.

[whitslack]

Not as convenient as allowing either form in the id parameter, as this means I'll have to type lightning-cli -k listpeerchannels short_channel_id=876543x21x0 instead of just lightning-cli listpeerchannels 876543x21x0. Of course, I could add an alias in my shell profile to save me the extra typing, but if I wanted to do that, I could have already implemented a jq filter atop the unparameterized listpeerchannels, which would have (mostly) obviated this feature request. Still, implementing the new parameter is a win because it can cut down on the volume of data that the RPC returns, so I thank you for your effort and very quick response time. 🍻

[rustyrussell]

Yes. Changing the type of an argument is problematic, because of grpc, which relies on the type. We get away with it for setchannel where we allow scid/peerid/channelid because we always did, and said it's just a "string" type.

lightning-cli listpeerchannels null 876543x21x0 works tho!

[whitslack]

lightning-cli listpeerchannels null 876543x21x0 works tho!

I didn't know positional arguments could be explicitly skipped by passing a literal 'null'. That completely addresses my complaint. Thank you!