Enable dependency isolation for Python function apps

[brettsam]

Discussion for #1969

[brettsam]

Notes from review meeting:

• This is the default already in 3.9. Ask is to make it default for all versions.
• Requires Host should pass capabilities (or version) to workers on startup azure-functions-host#7549.
• It would be good to gather logs (if possible) that would let us add a check to the migration detector. Is it possible to add additional logging now that would help with this?
• PYTHON_ISOLATE_WORKER_DEPENDENCIES -- this needs to have an inverse flag like PYTHON_DISABLE_ISOLATE_WORKER_DEPENDENCIES