If no `agentVersion` is provided for the Identify protocol, the default `AGENT_VERSION` will now be set to
* `js-libp2p/<libp2p.version> UserAgent=<process.version>` when running in Node.js
* `js-libp2p/<libp2p.version> UserAgent=<navigator.userAgent>` when running in the browser (also when running in a webworker)
Fixes#686
Supersedes #1240
Co-authored-by: Kevin Westphal <westphal@consider-it.de>
Co-authored-by: Kevin <56823591+6d7a@users.noreply.github.com>