feat: mount the whole repo in the pgai-ext container and add ai.version()

[jgpruitt]

We want to add a way to get the git commit hash of the extension's build from a SQL function. To do this, the .git directory must be inside the dev container. With this PR, we now mount the whole repo into the extension dev container. This lets us access git information during the build process.

Additionally, this allows us to access the examples directory from the dev container, which is helpful in testing the examples (especially the text-to-sql one).

I moved the .venv python virtual environment to /py/.venv inside the dev container. This is NOT mounted to the host. Thus, the .venv inside the container and the .venv on the host cannot possibly clash.