Add support for ansible-vault from virtual environment

[daincredibleholg]

I use virtual environments for my Ansible projects. This way, I can be very granular when it comes to versions and dependencies.
Anyways, while vscode lets me select the (auto-discovered) Python interpreter from the venv, ansible-vault seems to not have such an option. My current workaround is to change the executable path to the corresponding binary in the venv. But it would be really helpful to have this some kind of auto-discovered.

The ideal solution would be to have configuration flag to say: Use virtualenv if available and in the background, the project folder(s) is searched for the corresponding ansible-vault binary. If none is found, the default one is used.