Support specifying python version for policy packs.

[MitchellGerdisch]

Hello!

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Issue details

Currently, the policy engine uses the default python version set on the machine where it is running, or one can set PULUMI_PYTHON_CMD to point at the python command to use.
It would be useful to be able to specify the python version to use as part of the policy pack configuration - say in the PulumiPolicy.yaml file.

Affected area/feature

Python policies

[justinvp]

How exactly do we envision this working? We have a way of specifying the version (or min version) of Python to use and if the version of Python we're going to use doesn't match, we error?