Rollup merge of #126930 - Xaeroxe:file-checksum-hint, r=chenyukang

Add unstable support for outputting file checksums for use in cargo

Adds an unstable option that appends file checksums and expected lengths to the end of the dep-info file such that `cargo` can read and use these values as an alternative to file mtimes.

This PR powers the changes made in this cargo PR rust-lang/cargo#14137

Here's the tracking issue for the cargo feature rust-lang/cargo#14136.

Author: workingjubilee