Implement pip install --report <file>

[andrewjcg]

Support pips --report flag, producing a JSON encoded file with metadata
on the packages to be installed.

[zanieb]

Thanks for looking into this.

Can you provide some more context on the choices you've made here? What changes were made to uv internals to facilitate the feature and why? What differences are there from pip's output and interface, if any? Are there parts of pip's output format that are controversial or problematic? Are you planning to add tests?