Add IOTransport for separate reader/writer (#376)

This commit adds ability to create a mcp.Transport with specific
io.ReadCloser and io.WriteCloser.
Ii is needed when you need create an mcp.Client for mcp.Server launched
in separate process and you need to communicate with it with it's own
stdin/stdout.

mcp/transport.go

@@ -93,6 +93,18 @@ func (*StdioTransport) Connect(context.Context) (Connection, error) {
 	return newIOConn(rwc{os.Stdin, os.Stdout}), nil
 }
 
+// An IOTransport is a [Transport] that communicates over separate
+// io.ReadCloser and io.WriteCloser using newline-delimited JSON.
+type IOTransport struct {
+	Reader io.ReadCloser
+	Writer io.WriteCloser
+}
+
+// Connect implements the [Transport] interface.
+func (t *IOTransport) Connect(context.Context) (Connection, error) {
+	return newIOConn(rwc{t.Reader, t.Writer}), nil
+}
+
 // An InMemoryTransport is a [Transport] that communicates over an in-memory
 // network connection, using newline-delimited JSON.
 type InMemoryTransport struct {