add helpers to verify API without starting the binary

add helpers to verify API without starting the binary

In future, we want to have a separate PVE::CLI::$binname class
for each binary. We can then simply load that class to verify
the API:

 use PVE::CLI::pct;

 PVE::CLI::pct->verify_api();

or to generate the docs:

 PVE::CLI::pct->generate_pod_manpage();