Plugin for `uv` workspaces

The python dependency and project management tool [`uv`](https://docs.astral.sh/uv/) brings [workspaces](https://docs.astral.sh/uv/concepts/projects/workspaces/) to the python ecosystem, patterned after Cargo workspaces. Adding a `uv` workspace plugin to `release-please` would allow for independent versioning per member, bumping a workspace member's version only when necessary, like the existing workspace plugins do.

Currently, when I use `uv` workspaces I version the entire workspace together, using `extra-files` to keep the member-level `pyproject.toml` files in-sync with the top-level `pyproject.toml`. This is safe, but overkill; would be much nicer to have proper workspace awareness in `release-please`!