This adds the scaffolding for a `uv workspace metadata` command, as an
equivalent to `cargo metadata`, for integration with downstream tools. I
didn't do much here beyond emit the workspace root path and the paths of
the workspace members. I explored doing a bit more in #16638, but I
think we're actually going to want to come up with a fairly
comprehensive schema like `cargo metadata` has. I've started exploring
that too, but I don't have a concrete proposal to share yet.

I don't want this to be a top-level command because I think people would
expect `uv metadata <PACKAGE>` to show metadata about arbitrary packages
(this has been requested several times). I also think we can do other
things in the workspace namespace to make trivial integrations simpler,
like `uv workspace list` (enumerate members) and `uv workspace dir`
(show the path to the workspace root).

I don't expect this to be stable at all to start. I've both gated it
with preview and hidden it from the help. The intent is to merge so we
can iterate on it as we figure out what integrations need.