Add lazy_with_connector + Unix socket support

[iantay-ant]

Adds Http2Connection::lazy_with_connector and connect_with_connector as the generic transport escape hatch (same pattern as tonic's Endpoint::connect_with_connector). Callers provide a tower::Service<Uri> that yields any hyper::rt::Read + Write stream; the library boxes it internally and runs the h2 handshake.

lazy_unix / connect_unix are thin convenience wrappers demonstrating the pattern — they build a service_fn closure that dials tokio::net::UnixStream.

BoxedIo is un-gated from client-tls since the custom-connector path uses the same type erasure. The box allocation is once per connection establishment, not per request.

API

// Generic — any transport
pub fn lazy_with_connector<C>(connector: C, authority: Uri) -> Self
where
    C: tower::Service<Uri> + Send + 'static,
    C::Response: hyper::rt::Read + hyper::rt::Write + Send + Unpin + 'static,
    C::Error: Into<BoxError>,
    C::Future: Send + 'static,

// Convenience — Unix domain socket
#[cfg(unix)]
pub fn lazy_unix(path: impl Into<PathBuf>, authority: Uri) -> Self

Example

let conn = Http2Connection::lazy_with_connector(
    tower::service_fn(|_uri: Uri| async {
        let stream = tokio::net::UnixStream::connect("/tmp/app.sock").await?;
        Ok::<_, std::io::Error>(hyper_util::rt::TokioIo::new(stream))
    }),
    "http://localhost".parse().unwrap(),
);

[github-actions]

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[iantay-ant]

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