Support VirtualEnv creation flags (e.g. `--python=...`)

[andyk]

Flags that can be passed when creating a virtualenv should be supported.

I ran into this when i started an EC2 instance running a recent Ubuntu and ran:

cd example_agents/acme_dqn
agentos run agent

The VirtualEnv got created with Python 3.10.4 interpreter, which we don't support yet.

One solution would be to make VirtualEnv a PCS Component with attributes:

• python_version
• requirements_path

[nickjalbert]

I think I like the idea of explicitly representing the venv in the Component DAG. Where would it attach? The command? Or the root Component?

I guess the root Component would be the closest to the system we have now (and is convenient because the Component usually has the requirements.txt in its repo). The VirtualEnv spec would probably have a repo too, right?