Add MEP endpoint support

[christopherwharrop-noaa]

This update adds support for multi-user (templatable) Globus Compute endpoints. The CLI and API are updated to support life cycle management of those endpoints. This involves supplemental actions for configuring and starting endpoints that the native globus-compute-endpoint does not handle.

For example, configuration of multi endpoints handles:

1. Removal of the user mapping option and file
2. Customizing the jinja2 user config template
3. Adding PATH to the user environment config to ensure login paths are set correctly

The start of multi endpoints also handles:

1. Running the endpoint as a setsid daemon process
2. Adding the pid file to enable correct reporting of status and normal stopping of the endpoint

Closes #132. Closes #137.