crypto: Don't panic on unsupported key types

[lexnv]

This PR ensures that litep2p does not panic when decoding public keys from received TCP noise handshake.

The code operated under the assumption that only the ed25519 key is valid in the context of Substrate.
However, peers could still use a different key (rsa / ecdsa) and cause the code to panic.
In those cases, an error is returned which terminates the negotiation handshake.

Discovered during testing a sync node with litep2p backend on kusama as part of #83.

Version: 1.10.0-cd9d08d6311

   0: sp_panic_handler::set::{{closure}}
   1: std::panicking::rust_panic_with_hook
   2: std::panicking::begin_panic_handler::{{closure}}
   3: std::sys_common::backtrace::__rust_end_short_backtrace
   4: rust_begin_unwind
   5: core::panicking::panic_fmt
   6: <litep2p::crypto::PublicKey as core::convert::TryFrom<litep2p::crypto::keys_proto::PublicKey>>::try_from
   7: litep2p::crypto::PublicKey::from_protobuf_encoding
   8: litep2p::crypto::noise::parse_peer_id
   9: litep2p::transport::tcp::connection::TcpConnection::negotiate_connection::{{closure}}
  10: <tokio::time::timeout::Timeout<T> as core::future::future::Future>::poll
  11: <litep2p::transport::tcp::TcpTransport as litep2p::transport::Transport>::negotiate::{{closure}}
  12: <futures_util::stream::futures_unordered::FuturesUnordered<Fut> as futures_core::stream::Stream>::poll_next
  13: <litep2p::transport::tcp::TcpTransport as futures_core::stream::Stream>::poll_next
  14: <litep2p::transport::manager::TransportContext as futures_core::stream::Stream>::poll_next
  15: litep2p::transport::manager::TransportManager::next::{{closure}}
  16: <tokio::future::poll_fn::PollFn<F> as core::future::future::Future>::poll
  17: <sc_network::litep2p::Litep2pNetworkBackend as sc_network::service::traits::NetworkBackend<B,H>>::run::{{closure}}
  18: sc_service::build_network_future::{{closure}}::{{closure}}::{{closure}}
  19: <futures_util::future::poll_fn::PollFn<F> as core::future::future::Future>::poll
  20: <sc_service::task_manager::prometheus_future::PrometheusFuture<T> as core::future::future::Future>::poll
  21: <futures_util::future::select::Select<A,B> as core::future::future::Future>::poll
  22: <tracing_futures::Instrumented<T> as core::future::future::Future>::poll
  23: tokio::runtime::park::CachedParkThread::block_on
  24: tokio::runtime::context::runtime::enter_runtime
  25: <tokio::runtime::blocking::task::BlockingTask<T> as core::future::future::Future>::poll
  26: tokio::runtime::task::core::Core<T,S>::poll
  27: tokio::runtime::task::harness::Harness<T,S>::poll
  28: tokio::runtime::blocking::pool::Inner::run
  29: std::sys_common::backtrace::__rust_begin_short_backtrace
  30: core::ops::function::FnOnce::call_once{{vtable.shim}}
  31: std::sys::pal::unix::thread::Thread::new::thread_start
  32: <unknown>
  33: <unknown>


Thread 'tokio-runtime-worker' panicked at 'not implemented: unsupported key type', /home/ubuntu/.cargo/git/checkouts/litep2p-2515ad90543f141a/153d388/src/crypto/mod.rs:103

cc @dmitry-markin

[dmitry-markin]

Nice catch!