Improve working with raw SQL

[MuhannadAlrusayni]

(updated) This PR allow us to fetch T: FromQueryResult using T::find_by_statement() from raw sql:

// with this PR we can call `T::find_by_statement()` where `T: FromQueryResult` and return `T`
let res: Vec<SelectResult> = SelectResult::find_by_statement(Statement::from_sql_and_values(
    DbBackend::Postgres,
    r#"SELECT "cake"."name", count("cake"."id") AS "num_of_cakes" FROM "cake""#,
    vec![],
))
.all(&db)
.await?;

[tyt2y3]

@MuhannadAlrusayni thank you for the contribution.

On a second look I found that Sized is already a requirement of FromQueryResult, so I merged the method back into FromQueryResult.