Add running cargo-semver-checks on rust-libp2p to CI

[staniewzki]

PR to solve #178

As mentioned in #178, there should be some support for running checks on multiple crates in CI. I think that that could be adjusted in a follow up PR by using a matrix, as it would be easier to do when a specific crate is given, especially since there can be some dependencies needed (like protobuf-compiler in rust-libp2p)

[obi1kenobi]

Looks good! Broadly, it just would be nice to have some more documentation (in the form of comments) describing what issue this regression test protects against, how the specific project and commit were chosen, etc. That sort of thing will be useful to us in the future when adding more regression / integration tests like this, as well as if one day this regression test fails and we need to figure out what went wrong. Because I promise that a year from now, none of us will remember what the problem was that made us want to add this test, and those docs will be all we have left to remind us.

[obi1kenobi]

Consider leaving some documentation (as comments near here and near the top of the job) describing why this particular commit is interesting and worth testing?