Add JSON support to FromRow derive (#2121)

* Add `json` attribute to `FromRow` derive

* Add docs and fix formatting

sqlx-core/src/from_row.rs

@@ -1,5 +1,4 @@
-use crate::error::Error;
-use crate::row::Row;
+use crate::{error::Error, row::Row};
 
 /// A record that can be built from a row returned by the database.
 ///
@@ -210,6 +209,48 @@ use crate::row::Row;
 ///
 /// In MySql, `BigInt` type matches `i64`, but you can convert it to `u64` by `try_from`.
 ///
+/// #### `json`
+///
+/// If your database supports a JSON type, you can leverage `#[sqlx(json)]`
+/// to automatically integrate JSON deserialization in your [`FromRow`] implementation using [`serde`](https://docs.rs/serde/latest/serde/).
+///
+/// ```rust,ignore
+/// #[derive(serde::Deserialize)]
+/// struct Data {
+///     field1: String,
+///     field2: u64
+/// }
+///
+/// #[derive(sqlx::FromRow)]
+/// struct User {
+///     id: i32,
+///     name: String,
+///     #[sqlx(json)]
+///     metadata: Data
+/// }
+/// ```
+///
+/// Given a query like the following:
+///
+/// ```sql
+/// SELECT
+///     1 AS id,
+///     'Name' AS name,
+///     JSON_OBJECT('field1', 'value1', 'field2', 42) AS metadata
+/// ```
+///
+/// The `metadata` field will be deserialized used its `serde::Deserialize` implementation:
+///
+/// ```rust,ignore
+/// User {
+///     id: 1,
+///     name: "Name",
+///     metadata: Data {
+///         field1: "value1",
+///         field2: 42
+///     }
+/// }
+/// ```
 pub trait FromRow<'r, R: Row>: Sized {
     fn from_row(row: &'r R) -> Result<Self, Error>;
 }