Onion Routing for libp2p

This might be a duplicate of #200.

Proposal

I would like to propose defining and implementing a way of doing onion routing through libp2p. This is explicitly not about integrating onion routing based transports like Tor or I2P into libp2p but rather about a onion router in libp2p that can be used as well as a behaviour and a transport.

Since there are many different opinions on how to do onion routing right I would suggest only defining the onion routing itself, i.e. the key agreement as well as traffic forwarding. Other aspects like how the route is built, the list of available entry, exit and middle nodes is retrieved should be left to the user of libp2p as it's sometimes highly specific to the use case.
Furthermore there should be a way of letting behaviors know that they should use the onion router for something.

Related issues / PRs

• Onion Routing considerations for libp2p #200
• Tor and nim-libp2p status-im/nim-libp2p#765
• Tor and rust-libp2p libp2p/rust-libp2p#2899

Start by reviewing the proposal and the related onion-routing considerations in issue #200, then compare the linked Tor and rust-libp2p discussions. No implementation files or tests are named; the work would be complete only after the libp2p onion-routing key agreement, traffic forwarding, and behavior integration are defined and implemented.