async connections

[pv42]

Implements #256.
Adds asynchronous connect method async connect_async(address) as async version of connect(address).
For this the following is added when the tokio-1 feature is enabled:

• mod async_connection with connect_async(..)
• trait AsyncMavConnection as [#[async_trait] as an async verions of trait MavConnection
• AsyncFileConnection, AsyncTcpConnection, AsyncUdpConnection implementing AsyncMavConnection
• AsyncPeekReader as async verions of PeekReader
• read/write functions read_versioned_msg_async, read_versioned_msg_async_signed, read_v1_raw_message_async, read_v1_msg_async, read_v2_msg_async_signed, write_versioned_msg_async_signed, write_v2_msg_async_signed
• change functions read_v2_raw_message_async, read_v2_raw_message_async_inner, read_v2_raw_message_async_signed, read_v2_msg_async to read from a AsyncPeekReader instead of directly for a tokio::io::AsyncReadExt
• connect_async is exposed as mavlink::connect_async

Tests:

• Add async versions of tcp_loopback_tests.rs and udp_loopback_async_tests.rs
• AsyncUdpConnection tests datagram_buffering similar to its blocking sibling
• To run test the tokio features rt, macros are needed, for this reason tokio with these additional features is required as dev-dependencies for both mavlink-core and mavlink

Caveats

• No serial support.
• The change of some existing function to AsyncPeekReader from tokio::io::AsyncReadExt is breaking
• The MSRV test fails (see Test with no-dev-deps all-targets #277/Run MSRV check only for lib and bin targtets. #278), test succeds locally for bins/lib targets or without using no-dev-deps.
• The combinations of asnyc and signing results in 4 versions of the same functions that effectifly do the same. (e.g. write_versioned_msg, write_versioned_msg_signed, write_versioned_msg_async and write_versioned_msg_async_signed)

[patrickelectric]

Nice!