Tracking Issue for workspace `feature-unification`

Summary

RFC: #3692 Implementation:

• feature-unification = "workspace" (#15157)
• feature-unification = "package" (#15684)

Documentation: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#feature-unification

Adds the resolver.feature-unification configuration option to control how features are unified across a workspace.

Unresolved Issues

• Bikeshed the config option naming.
• for feature-unification = "package", what should cargo tree look like (https://github.com/rust-lang/cargo/pull/15684#discussion_r2159629336)

Future Extensions

• Support in manifests
• Dependency version unification
• Unify features in other settings

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.