Support mutable method arguments with dbus_interface macro

Currently it's possible to directly declare a method argument as mutable when using the dbus_proxy macro, but when using dbus_interface it gives an error, that disappears when dropping the dbus_interface macro:

error: expected expression, found keyword `mut`
   --> src/example.rs:189:23
    |
189 |     async fn t(&self, mut s: String) -> fdo::Result<()> {
    |                       ^^^ expected expression

error: expected expression
   --> src/example.rs:189:23
    |
189 |     async fn t(&self, mut s: String) -> fdo::Result<()> {

with example code

struct Test;
#[dbus_interface(name = "org.example.Test")]
impl Test {
    async fn t(&self, mut s: String) -> fdo::Result<()> {
        s.push_str("foo");
        Ok(())
    }
}

Of course in this case it's possible to clone and shadow the argument, but it's less convenient and wastes memory (in the case where borrowed arguments are used).