feat: extensible peer ids

[achingbrain]

Sometimes a peer will have a non-cryptographic identifier such as a URL.

Update the PeerId type to be an interface so we can use arbitrary data to identify other peers, not just cryptographic keys.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[SgtPooki]

two minor issues in comments. otherwise looks pretty straightforward.