Allow extensions to contribute environment creation providers and trigger environment creation

[karthiknadig]

Adds following features to the python extension environments namespace:

1. A function to trigger environment creation.
2. A function to register an environment creation provider.
3. An event to indicate environment creation has started.
4. An event to indicate environment creation is done.

https://github.com/microsoft/vscode-python/blob/main/src/client/pythonEnvironments/creation/proposed.createEnvApis.ts