When libp2p is [configured with arbitrary services](libp2p/js-libp2p#1563)
some of those services may be content routers or peer routers or
both.  An example of this is the `@libp2p/kad-dht` module.

In order to communicate to libp2p that they can provide content/peer
routing cabapiliy, add well-known symbols that libp2p can use to
get references to the routing implementations.