Use github actions annotations for warnings

[LucioFranco]

This github action should take advantage of the annotations feature for making it easier for users to find the issues when the action fails. I think if the Cargo.toml is present in the diff then you can add the annotation there. If it is not it could just be appended to the end of the diff.

A good reference is the https://github.com/actions-rs/clippy-check action which uses this feature.

[Jake-Shadle]

Unfortunately annotations are limited to files that actually exist in the repository the action is executed on, but there is the possibility to add a markdown file as the summary for the entire job that supports full github markdown, so adding output/graphs here seems to be the way forward.

Related: EmbarkStudios/cargo-deny#433

[adam-azarchs]

If your PR adds a new dependency which triggers an issue, the annotation could be on the line in your Cargo.toml which adds the dependency. This isn't super helpful for transitive dependencies, nor is it all that helpful for lockfile-only updates - you could put the annotations in Cargo.lock, if present, but GitHub suppresses the diffs there by default.