Specializing trait methods for given cases

Thank you for the nice crate! I needed support for specializing some method implementations in the generated traits for each case, while having a single implementation for the rest of the trait. What I came up with is a `#[when]` attribute that can be used on methods within a `#[trait_gen]` trait, for example ```rust #[trait_gen(DB -> sqlx::sqlite::Sqlite, sqlx::mysql::MySql, sqlx::postgres::Postgres)] #[async_trait] impl Repo for sqlx::Pool<DB> { // ... #[when(sqlx::sqlite::Sqlite -> update)] async fn update_sqlite(&self, id: u64, param: String) -> Result<(), String> { // SQLite specific implementation todo!() } #[when(sqlx::mysql::MySql -> update)] async fn update_mysql(&self, id: u64, param: String) -> Result<(), String> { // MySQL specific implementation todo!() } // ... ``` As I needed this quickly, I published it as a fork: https://github.com/vigoo/conditional_trait_gen But if you are interested, I'm happy to open a PR with it upstream.