[OpenAPI] Support using Default trait when derive Object

[duskmoon314]

Description of the feature

Currently, only the fields of Object has the default parameter. And it can only be used for some types that already implement Default, or have a function to generate the default value.

It might be more convenient if we could define Default for structs and use it.

Notice: I don't know if this is possible. Have taken a simple look at the macro of Object, it takes fields' attributes one by one and generates codes.

Code example (if possible)

Suppose rust codes like this:

#[derive(Object)]
#[oai(default)]
struct MyObj {
  str: String,
  int: isize
}

impl Default for MyObj {
  fn default() -> Self {
    Self {
      str: "some_string".to_string(),
      int: 32,
    }
  }
}

[sunli829]

Absolutely, I'll add it later.

[sunli829]

Released in v2.0.10

poem/poem-openapi/tests/object.rs

Line 823 in 05d568e

#[oai(default)]

[duskmoon314]

Amazing! Thanks!