Extend DeriveIntoActiveModel to support settable and default-able values

[sinder38]

Motivation

When using DeriveIntoActiveModel for my create and patch API/Serde endpoint input structs, there is no way to express:

1. “If the user didn’t provide this field, use my custom default.”
2. “Always use this set value for this required field.”

Currently, this requires either a manual implementation or explicitly setting those values every time the DeriveIntoActiveModel struct is used.

Example model that would benefit from this feature:

#[sea_orm::model]
#[derive(Clone, Debug, PartialEq, Eq, DeriveEntityModel, Serialize, Deserialize)]
#[sea_orm(schema_name = "public", table_name = "file")]
pub struct Model {
    #[sea_orm(primary_key, auto_increment = false)]
    pub id: I32,
    pub creator_id: i32,
    pub created_at: DateTime<Utc>, // <- set using current_time function
    pub updater_id: Option<i32>,
    pub updated_at: Option<DateTime<Utc>>,
    pub deleted: bool, // <- set using my own default every time
}

Proposed Solutions

I will link my PR below with more details.

Additional Information

I also believe the documentation for this derive macro is lacking.