feat: add custom socket transport support for Postgres and MySQL

Add methods to PgConnection and MySqlConnection that accept pre-connected
sockets implementing AsyncRead + AsyncWrite, enabling custom transport
layers (vsock, QUIC, turmoil, SSH tunnels, etc.) without forking sqlx.

Per maintainer feedback on #4187, this uses AsyncRead + AsyncWrite traits
instead of exposing the internal Socket trait. Two separate methods are
provided for each runtime's trait set:

- connect_with_custom_tokio(): accepts tokio::io::{AsyncRead, AsyncWrite}
- connect_with_custom_futures(): accepts futures_io::{AsyncRead, AsyncWrite}

Also adds PoolOptions::connector() so pools can use custom transports:

    PgPoolOptions::new()
        .connector(|options| async move {
            let socket = VsockStream::connect(addr).await?;
            PgConnection::connect_with_custom_tokio(socket, &options).await
        })
        .connect_with(options)
        .await?

TLS upgrade is negotiated automatically based on the connection options.
No new public trait exposure. No behavioral changes to existing code.

Author: Benjscho

Cargo.toml

@@ -94,10 +94,10 @@ _unstable-docs = [
 ]
 
 # Base runtime features without TLS
-runtime-async-global-executor = ["_rt-async-global-executor", "sqlx-core/_rt-async-global-executor", "sqlx-macros?/_rt-async-global-executor"]
-runtime-async-std = ["_rt-async-std", "sqlx-core/_rt-async-std", "sqlx-macros?/_rt-async-std"]
-runtime-smol = ["_rt-smol", "sqlx-core/_rt-smol", "sqlx-macros?/_rt-smol"]
-runtime-tokio = ["_rt-tokio", "sqlx-core/_rt-tokio", "sqlx-macros?/_rt-tokio"]
+runtime-async-global-executor = ["_rt-async-global-executor", "sqlx-core/_rt-async-global-executor", "sqlx-macros?/_rt-async-global-executor", "sqlx-postgres?/_rt-async-io", "sqlx-mysql?/_rt-async-io"]
+runtime-async-std = ["_rt-async-std", "sqlx-core/_rt-async-std", "sqlx-macros?/_rt-async-std", "sqlx-postgres?/_rt-async-io", "sqlx-mysql?/_rt-async-io"]
+runtime-smol = ["_rt-smol", "sqlx-core/_rt-smol", "sqlx-macros?/_rt-smol", "sqlx-postgres?/_rt-async-io", "sqlx-mysql?/_rt-async-io"]
+runtime-tokio = ["_rt-tokio", "sqlx-core/_rt-tokio", "sqlx-macros?/_rt-tokio", "sqlx-postgres?/_rt-tokio", "sqlx-mysql?/_rt-tokio"]
 
 # TLS features
 tls-native-tls = ["sqlx-core/_tls-native-tls", "sqlx-macros?/_tls-native-tls"]

sqlx-core/src/net/mod.rs

@@ -4,3 +4,9 @@ pub mod tls;
 pub use socket::{
     connect_tcp, connect_uds, BufferedSocket, Socket, SocketIntoBox, WithSocket, WriteBuffer,
 };
+
+#[cfg(feature = "_rt-tokio")]
+pub use socket::async_rw_adapter::TokioStream;
+
+#[cfg(feature = "_rt-async-io")]
+pub use socket::async_rw_adapter::FuturesStream;