aip/4114: call the cert_provider_command

[coryan]

Once the context_aware_metadata.json file is parsed one needs to invoke a program to get the client certificate.

• [ ] Implement a helper to invoke programs on *nix systems and capture their output
• [ ] Implement a helper to invoke programs on Windows systems and capture their output

https://google.aip.dev/auth/4114

[coryan]

We should do this at some point.

[coryan]

Strike 2, we still want to do this.

[coryan]

We do not have time to work on this, closing.