feat: complete `Service` client implementation

[Dan Cline]

Currently service.rs contains a mostly unimplemented Service on the server side. The tower Service abstraction is convenient for both clients and servers, so a client should be easy to implement for a single peer.

Once implemented, dialing a node should look something like this:

let mut peer = make_client(peer_addr).await.unwrap();

The client should also make p2p requests more ergonomic. For example rather than dealing with peer message channels directly, we should have the following UX for making requests:

// using From<Vec<[u8; 32]>> for GetPooledTransactions
let txs_request: GetPooledTransactions = vec![
    hex!("c05a9eda51dd448c79e1dda1c79a3835c82c1142104f88adc1a64ec80fd193fb"),
    hex!("2fa4c5912523f735581cc731a9d44870fbb45cc3cbfb529d643b5aaca112beda"),
    hex!("3f855459086213a3def40270ee1c6e12aba7f5f81375ca35cdcd8e71e7a9b092"),
].into();

// a lower-level way to make a request, using the `Service` trait, still not bad
let request = Request::GetPooledTransactions(
    RequestPair {
        request_id: 1,
        message: txs_request,
    }
);
let response = peer.call(request).await.unwrap();

// another possible way to make a request
// if using the `Service` under the hood the implementation of this method should be very easy
let txs_response = peer.get_pooled_transactions(txs_request).await.unwrap();