Add `python-downloads-json-url` option for `uv.toml` to configure custom Python installations via JSON URL (#12974)
## Summary
Part of #12838. Allow users to configure `python-downloads-json-url` in
`uv.toml` and not just from env.
I followed similar PR #8695, so same as there it's also available in the
CLI (I think maybe it's better not to be configurable from the CLI, but
since the mirror parameters are, I think it's better to do the same)
## Test Plan
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