Source-based coverage

[davidhewitt]

Motivation

I'd love to see support for the new source-based profiling as an action, similar to grcov and tarpaulin actions.

Additional context

See https://blog.rust-lang.org/inside-rust/2020/11/12/source-based-code-coverage.html

[Swatinem]

I was playing around with this recently and created https://github.com/Swatinem/fucov#fucov, although all of this is still experimental I would say.