R1:4236952cdbc0

Add a `bin/config set <key> --stdin < value.json` flag to make CLI configuration of complex values easier

Summary:
Depends on D19003. Ref T12677. Ref T13053. For the first time, we're requiring CLI configuration of a complex value (not just a string, integer, bool, etc) to do something fairly standard (send mail).

Users sometimes have very reasonable difficulty figuring out how to `./bin/config set key <some big JSON mess>`. Provide an easy way to handle this and make sure it gets appropriate callouts in…