msggen: add missing-grpc command

[jackstar12]

This PR adds a missing-grpc command to msggen which can be used to track down missing grpc methods: #7167 (comment)
It currently outputs:

Missing RPC commands in grpc:
delforward
makesecret
reserveinputs
batching
upgradewallet
openchannel_signed
dev-forget-channel
renepaystatus
autoclean-status
delpay
fundchannel_complete
bkpr-listaccountevents
emergencyrecover
check
sql-template
splice_init
commando-blacklist
openchannel_abort
showrunes
getlog
listinvoicerequests
parsefeerate
multiwithdraw
openchannel_init
disableinvoicerequest
splice_signed
disableoffer
commando-listrunes
notifications
openchannel_bump
bkpr-dumpincomecsv
autoclean-once
checkrune
recoverchannel
fundchannel_start
deprecations
splice_update
bkpr-channelsapy
blacklistrune
sendonionmessage
recover
addpsbtoutput
unreserveinputs
listconfigs
plugin
createrune
fundchannel_cancel
commando-rune
listsqlschemas
commando
help
multifundchannel
renepay
openchannel_update
funderupdate
bkpr-inspect
bkpr-listbalances
sendinvoice
invoicerequest
setpsbtversion
setconfig

[cdecker]

ACK 3b64d42

Building tooling around a temporary thing may appear weird, and I thought so too initially, but "temporary" had been the status quo for too long not to do this 😅