derive FromRow: sqlx(default) for all fields

[grgi]

This commit adds sqlx(default) as a struct attribute when deriving FromRow - imitating the similar behaviour of serde(default). E.g. the following two definitions are equivalent:

#[derive(sqlx::FromRow)]
#[sqlx(default)]
struct Options {
    option_a: Option<String>,
    option_b: Option<i32>,
}

derives the same FromRow implementation as

#[derive(sqlx::FromRow)]
struct Options {
    #[sqlx(default)]
    option_a: Option<String>,
    #[sqlx(default)]
    option_b: Option<i32>,
}

[abonander]

Strictly speaking, if you want it to be equivalent to #[serde(default)] then it should actually require the annotated struct itself to implement Default: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=7fb4f62b44e7e63acc2e1cfe1599a59a

It actually constructs the struct with Default::default() and takes fields from it if they don't appear in the input.

The two approaches are equivalent if #[derive(Default)] is also used on the struct itself, since that fills all the fields with Default::default() anyway, but a manual Default impl can change that behavior.

Because of that it's probably the more flexible and less surprising option, although it does preclude implementing Drop for the struct since you can't move non-Copy fields from a struct that implements Drop.

[grgi]

Thank you for the suggestion! I have changed it accordingly, so now it requires Default and moves the values from the default implementation if missing from the query.

[abonander]

Just some copy-editing for the docs.