add a function to set the UDP send buffer size

This function is the equivalent to the function used to set the UDP
receive buffer size. It's so similar that code generation is used to
make a copy of the setReceiveBuffer function.

sys_conn_helper_nonlinux.go

@@ -2,4 +2,5 @@
 
 package quic
 
-func forceSetReceiveBuffer(c interface{}, bytes int) error { return nil }
+func forceSetReceiveBuffer(c any, bytes int) error { return nil }
+func forceSetSendBuffer(c any, bytes int) error    { return nil }