feat(register): add output dir for credentials

[bassosimone]

This patch adds a new flag: -output-secure. Its default value is the value used by -output, for backward compatibility with deployments that only specify the -output flag.

The purpose of this patch is to allow a consumer like byos-debian to emit the service account key (and possibly other secrets in the future) into distinct dirs with distinct permissions.

In turn, this ensures that network measurement services such as the ndt-server cannot read the service account key, which is anyway a secret it should not have access to, while still being able to source the public files it requires.

We named the flag -output-secure= (as opposed to something like -secure-output=) because we want it to sort near the -output= flag for additional -h/-help clarity.

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This change is 

[robertodauria]

-output-sec read as output seconds to me at first. Perhaps -output-secrets?

[bassosimone]

I discussed this with @robertodauria and he suggested changing the flag to -output-secur=. We also discussed whether -secure-output= would have been more idiomatic. Given that -output= already exists, I suggested keeping -output-secure= so they sort together in the CLI.