support flatten attribute in FromRow macro

[TheoOiry]

Adding #[sqlx(flatten)] field attribute in a FromRow macro to handle a field that implements FromRow

This is a task of #156 : "Handle a field that is impl FromRow. A parallel from serde_json would perhaps make this #[sqlx(flatten)]"

[abonander]

Do you have the time to write documentation for this attribute here:

sqlx/sqlx-core/src/from_row.rs

Line 22 in d9fd21c

/// ### Field attributes

[TheoOiry]

Do you have the time to write documentation for this attribute here:

sqlx/sqlx-core/src/from_row.rs

Line 22 in d9fd21c

/// ### Field attributes

My bad, I forgot about that. It's done.

[abonander]

Very good, thank you!