Added missing special casing for encoding embedded arrays of custom types

[nico-incubiq]

fixes #1672 - or at least my last comment on the ticket, as I now realise the person originally reported a decoding issue, and this is about encoding, as decoding already worked fine. My specific scenario has been added to the test suite to confirm this fixes it.

This fix is inspired by this similar bit of code:
https://github.com/launchbadge/sqlx/blob/82d332f4b487440b4c2bd5d54a5f17dcc1abc92c/sqlx-postgres/src/types/array.rs#L168-L175

With this fix in, it's now perfectly fine to store (and retrieve) nested structures in Postgres, without any additional boilerplate, such as:

#[derive(Debug, sqlx::Type)]
#[sqlx(type_name = "http_response")]
struct HttpResponseRecord {
    status_code: i16,
    headers: Vec<HeaderPairRecord>,
    body: Vec<u8>,
}

#[derive(Debug, sqlx::Type)]
#[sqlx(type_name = "header_pair")]
struct HeaderPairRecord {
    name: String,
    value: Vec<u8>,
}

[...]

sqlx::query!(
    r#"
    INSERT INTO idempotency (user_id, idempotency_key, response, created_at)
    VALUES ($1, $2, $3, NOW())
    "#,
    &user_id,
    &idempotency_key,
    &http_response_record as _,
)
.execute(connection)
.await?;

let saved_response = sqlx::query!(
    r#"
    SELECT response AS "response: HttpResponseRecord"
    FROM idempotency
    WHERE user_id = $1
    AND idempotency_key = $2
    "#,
    &user_id,
    &idempotency_key
)
.fetch_optional(connection)
.await?;

By the way, I've seen comments pointing to using the newtype pattern to map embedded arrays of custom types; but it's no longer necessary after this fix, so it might be worth updating the official documentation with an example.