We are inferring the field numbers on the fly, which isn't really
compatible with the way GRPC field numbers work, i.e., they must be
stable while the IDL file evolves. So far when a field was added in
the middle of a struct or removed all subsequent fields would get
renumbered, essentially breaking any client that was using the old
scheme.
We now add a meta file `.msggen.json` that keeps track of the numbers
assigned so far, so they can be reused, and new ones can be generated
not to conflict with existing ones. This file is intentionally kept
generic, so other generators can add more information that has to be
managed across runs.
Changelog-None
