Update manifest.json Python version population

[sagerb]

We should update how we decide which Python version we specify for running on Connect. This work would update the the Python.version field in the config to be optional, and document what it means to set it. This should also update how we build the manifest.json accordingly:

From the Python.version in the config, if set (this field is now optional)
From the current active version of Python in the IDE, if the Python.version is not set in config
The answer to this should populate thepython.version field in the manifest.json, when we build it.

Note that this doesn't lean on the .python-version file written by pyenv (docs). If we tackle that change, it will be in a follow-on issue.