feat: add ValueStore interface

[yusefnapora]

This adds a ContentRoutingStorage interface with put and get methods for storing key/value data either with the DHT or via delegated routing (and maybe pubsub? I'm not sure if pubsub routing has k/v storage).

The idea is that instead of directly hitting the dht in js-libp2p here, we can loop through all the routers and do something like this:

    // try using all routers that support the `put` operation.
    // if more than one support it, the first to resolve will be used
    const promises = []
    for (const router of this.routers) {
      if ('put' in router && typeof router.put === 'function') {
        promises.push(router.put(key, value, options))
      }
    }
    
    return raceToSuccess(promises)

Where raceToSuccess returns the first promise to resolve, or fails if all promises fail. The DHT already conforms to this new interface, and libp2p/js-libp2p-delegated-content-routing#54 will add it to the delegated routing module.

Open to suggestions on the interface name 😄

[yusefnapora]

I realized that the dht doesn't return raw Uint8Arrays from get - instead it returns objects like

{
  from: PeerId,
  val: Uint8Array
}

So the last commit adds that to the interface.

[vasco-santos]

This looks good from the interface perspective. Thanks @yusefnapora

We should create an issue to track further improvements by adding the actual test suite