Adding webrtc-based example

[plauche]

No description provided.

[plauche]

I'm having some issues getting the import for the webrtc transport to work.

I started with:

$ go build
main.go:19:2: no required module provides package github.com/plauche/go-libp2p/p2p/transport/webrtc; to add it:
	go get github.com/plauche/go-libp2p/p2p/transport/webrtc

So I tried to add it using Chinmay's fork/branch:

$ go get github.com/ckousik/go-libp2p/p2p/transport/webrtc@ckousik/webrtc
go: github.com/ckousik/go-libp2p/p2p/transport/webrtc@ckousik/webrtc: invalid version: version "ckousik/webrtc" invalid: disallowed version string

Then I made my own fork with a more friendly branch name, webrtc, and tried again:

$ go get github.com/plauche/go-libp2p/p2p/transport/webrtc@webrtc
go: github.com/plauche/go-libp2p@v0.20.1-0.20221012133416-6c3bf8a530a6: parsing go.mod:
	module declares its path as: github.com/libp2p/go-libp2p
	        but was required as: github.com/plauche/go-libp2p

And now it seems unhappy that I'm working off a fork.