Rollup publish emails for workspace publishes

[jschwe]

This was mentioned before in #9355, but only an opt-out got implemented.
When publishing a large workspace, it would be nice to have an option to receive 1 (or 2 emails, e.g. one to notify that the publish is starting, and one when the workspace publish is done).

For large workspaces, the emails are pure noise, since nobody is going to check 30+ emails, and verify that nothing snuck in. An attack that gained access to some crate, could observe the CI publishing workflow is running, and time their publish of a compromised crate (of the same owner, but from a different repository) to match the publish workflow, so that the email notification drowns in the noise of all the legitimate publishes.

If cargo publish --workspace publishing got a rolled up email, then we could keep the security benefit of notifications, without drowning in noise. Since many projects with larger workspaces don't use cargo publish --workspace, since it doesn't support resuming after errors, probably there should be also an easy mechanism to get rollup behavior for custom cargo publish wrappers for workspaces.