When the user passes in --runtime-python-path and --runtime-env-externally-managed that means that the python environment is wholly maintained outside of runt.

This is the simplest scenario for running a python kernel, since there's no initialization or lifetime issues with the environment.

Instead the python-runtime-agent should:

• Ensure that the python path is valid
• Launch the kernel, rooted at the environment, and using a temporary location for conn.json
• Bind the cell execution calls to the kernel
• Shutdown the process during onShutdown

Don't worry about discovering kernels, or using kernel.json that jupyter uses. We can cross that bridge later, if necessary