`uv python pin` should support writing to a `uv.toml` or `pyproject.toml`

[zanieb]

We need to design the schema for this data. Probably at uv.python-version or uv.python.default-version.

[T-256]

Then, should it also be top-level command uv pin? since it tries to manage project/workspace directly.

[zanieb]

That's a reasonable question. I'm not sure. I don't love it at the top-level since it's not obvious what we're pinning.

[zanieb]

I duped myself, #4359 covers reading from the configuration file.

[charliermarsh]

(At least figure out the rough design prior to the release.)