ci: add github-actions ecosystem to dependabot

[cpu]

👋 It looks like you beat me to this repo for the majority of the CI update tasks. Thanks!

The only omission I noticed that might be worthwhile was configuring dependabot for github actions updates If you wanted to avoid that on purpose for some reason that's OK too, please close as required.

[chifflier]

Indeed, I tried updating the CI in this crate (if you have feedback on the resulting rust.yml do not hesitate to comment), and it seems to work fine.
I am wondering if I should add feature test matrix for more cases (like clippy), since for example I forgot to add --all-features, and it lead to catching a warning (fixed in 2d84192).

The jobs can be a bit painful though, because it upgrades clippy to nightly, and several warnings were added since stable.
That said, it's a good thing :)

[chifflier]

Merged, thanks!

[cpu]

I am wondering if I should add feature test matrix for more cases (like clippy),

SGTM. I left a comment on the pcap-parser PR suggesting we crib from this repo. If that works for you I'll update the others.