msggen: add `missing-grpc` command #4

Job	Run time
Pre-build checks	3m 39s
Run fuzz regression tests	0s
Compile CLN ${{ matrix.cfg }}	0s
Test CLN ${{ matrix.name }}	0s
Sanitizers Test CLN	0s
Valgrind Test CLN ${{ matrix.name }}	0s
Run unit tests	0s
CI completion	0s
	3m 39s

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