feat: new --root-only flag for metadata

[tkr-sh]

This PR introduces a new flag for cargo metadata: --root-only.

This flag will only output the cargo workspace root.

This is a useful information. But when you're in a big workspace, it can take a lot of time just to get the workspace root. Therefore, this flag allow users to get the workspace without computing all the workspace's metadata.

[rustbot]

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[weihanglo]

As a heads up, our contribution process asks that issues be labelled as S-accepted before moving on to PRs.

[weihanglo]

To find the workspace root, there's already a similar command cargo locate-project

[tkr-sh]

@weihanglo Oh Sorry! I missed that, thanks! (Opened: #15106)

[tkr-sh]

oh... thanks! Going to close this PR therefore!