This adds parsing if the `secret_file` from YAML. And reads the
contents of the configured file.

If no file is configured, we fall back to the `.gitlab_shell_secret`
in the root dir of the configuration.

If the configured path does not start with a `/` we start looking for
the file relative to the root dir of the configuration.

Adds distributed tracing instrumentation to GitLab-Shell using LabKit

This reverts merge request !262

The client now has TLS support. So this gets pulled into this component.
To make it work, we have to upgrade the proto too.