feat: add Connection::from_reader_writer for pre-connected transports

[lsjostro]

Add a public constructor that builds a Connection from an already-connected reader/writer pair, rather than resolving a varlink URI and opening the socket internally.

Motivation

Connection can currently only be constructed from a varlink address URI (with_address/with_activate/with_bridge) or by spawning a child. There is no supported way to drive the protocol over a transport the URI scheme does not cover — e.g. an AF_VSOCK socket, an anonymous pipe, or a pre-established socketpair.

Today the only options are to carry a downstream patch or build the Connection struct by literal (impossible externally, since the address field is private). We currently maintain exactly such a patch to talk to a VM guest over AF_VSOCK; this upstreams it so the use case is supported without forking the crate.

What this adds

pub fn from_reader_writer<R, W>(reader: R, writer: W) -> Arc<RwLock<Self>>
where
    R: Read + Send + Sync + 'static,
    W: Write + Send + Sync + 'static,

The caller owns connecting the transport; varlink only reads from reader and writes to writer. The resulting connection holds no address and no owned stream, so the underlying transport is not closed on drop — that stays the caller's responsibility. It then works with the generated VarlinkClient like any other Connection.

Testing

• cargo build -p varlink — clean
• cargo test -p varlink --doc — passes (incl. the new doctest)
• cargo fmt -p varlink -- --check and cargo clippy -p varlink — clean

[haraldh]

Thank you!