Allow usage of generics in dbus_proxy trait

Currently, trying to write such Proxy

#[derive(Serialize_repr, Deserialize_repr, PartialEq, Debug, Type)]
#[repr(u32)]
enum ResponseType {
    /// Success, the request is carried out
    Success = 0,
    /// The user cancelled the interaction
    Cancelled = 1,
    /// The user interaction was ended in some other way
    Other = 2,
}

#[dbus_proxy(default_path = "/org/freedesktop/portal/desktop")]
trait Request {
    #[dbus_proxy(signal)]
    /// A signal emitted when the portal interaction is over.
    fn response<T>(&self, response: (ResponseType, T)) -> Result<()>
    where
        T: serde::de::DeserializeOwned + zvariant::Type;

    /// Closes the portal request to which this object refers and ends all related user interaction (dialogs, etc).
    /// A Response signal will not be emitted in this case.
    fn close(&self) -> Result<()>;
}

Fails with the current issue

error[E0412]: cannot find type `T` in this scope
  --> src/request.rs:60:52
   |
56 | #[dbus_proxy(default_path = "/org/freedesktop/portal/desktop")]
   | --------------------------------------------------------------- similarly named type parameter `H` defined here
...
60 |     fn response<T>(&self, response: (ResponseType, T)) -> Result<()>
   |                                                    ^ help: a type parameter with a similar name exists: `H`

warning: unused import: `ObjectPath`
 --> src/request.rs:5:16
  |
5 | use zvariant::{ObjectPath, OwnedValue};
  |                ^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

error[E0282]: type annotations needed
  --> src/request.rs:60:27
   |
60 |     fn response<T>(&self, response: (ResponseType, T)) -> Result<()>
   |                           ^^^^^^^^ consider giving `response` a type

error: aborting due to 2 previous errors; 1 warning emitted

Some errors have detailed explanations: E0282, E0412.
For more information about an error, try `rustc --explain E0282`.
error: could not compile `ashpd`