pw_system: Add user-specified protos to console entry point
pw_system: Add user-specified protos to console entry point

Adds a mechanism for users of pw-system-console to inject compiled
protos into an invocation of pw-system-console when using their own
python program's main() entrypoint as a trivial wrapper around
pw_system.console.main()

This is useful for pw_system users who have some custom RPC services,
but do not (yet) need a fully-forked console implementation.

Bug: pwbug/647

Change-Id: If21f999a41dfc76a8a271e653565ab4d1b4f3e34
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/90500
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed