move codegen to xtask

xtask/src/main.rs

@@ -0,0 +1,22 @@
+mod codegen;
+
+fn main() {
+    let command = std::env::args().nth(1).unwrap_or_else(|| {
+        eprintln!("No task provided");
+        help();
+        std::process::exit(1);
+    });
+
+    match command.as_str() {
+        "generate" | "gen" => codegen::main(),
+        _ => eprintln!("Unknown command {command}."),
+    }
+}
+
+fn help() {
+    println!(
+        "Available tasks:
+generate - Generate amqp method code in `amqp_transport/src/classes/generated.rs.
+           Dumps code to stdout and should be redirected manually."
+    );
+}