Rust: Allow for stable identities that don't depend on the file-system

[thomaseizinger]

With #91, we are continuously deploying the Rust peer to fly.io. That is great except that at the moment, the identity and the WebRTC certhashes change with every deployment.

We should change the code such that we can control these from a seed that we can specify in an environment variable.

[thomaseizinger]

It might be best to put the entire cert in an env var for now as we can't control the randomness for a new cert very well in the current WebRTC implementation.

See libp2p/rust-libp2p#3049.