feat: allow passing extra config k,v pairs for pyvenv.cfg when creating a venv

[samypr100]

Summary

This modifies gourgeist to allow passing additional k,v pairs to add to the pyvenv.cfg file as proposed in #1697.
I made it allow an arbitrary set of pairs (to decouple from uv since this is mainly a change to gourgeist) , but I can slim it down to just allow just a name and version strings if that's desired.

The pyvenv.cfg will also have a uv = <uv-crate-version> when a venv is created via uv venv and uv-build = <uv-build-crate-version> when it's created via SourceBuild::setup.

Example below via uv venv:

home = ...
implementation = CPython
version_info = 3.12
include-system-site-packages = false
base-prefix = ...
base-exec-prefix = ...
base-executable = ...
uv = 0.1.6
prompt = uv

Open to any suggestions, thanks!

Closes #1697

Test Plan

Added new test in tests/venv.rs called verify_pyvenv_cfg to verify that it contains the right uv version string. I didn't see tests configured in gourgeist itself, so I didn't add any there.

[konstin]

Thanks!

[henryiii]

Shouldn’t gourgeist still be there too? I am checking for that in nox (though not merged yet due to the coverage CI being broken). I can check for either one easily enough.

[konstin]

I think the "API" should be uv with gourgeist being more an implementation detail. What's your use case for checking who created the venv?

[samypr100]

@henryiii Can you look for uv starting with 0.1.9 instead? I believe that's what you desired for nox based on your issue.

[henryiii]

I can look for either one. I need to know if we need to regenerate the environment if a user changes the provider. This is especially important for uv vs. venv/virtualenv, since we are installing pip into those environments but not uv.

[henryiii]

Technically, since it didn't matter before (unless someone switched venv or virtualenv with conda or mamba, which would have been a mess but not something very common), the mechanism is weirdly opt-in via a secret environment variable, but I think I can fix that.