feat: AWS RDS

[chesedo]

Adds support for Postgres, Mysql and MariaDB AWS RDS

Needs #167

Breaking change

All old deploys that use the PgPool needs to add attribute configuration to explicitly ask for a shared Postgres resource as follow:

#[shuttle_service::main]
async fn rocket(#[shared::Postgres] pool: PgPool) -> Result<Rocket<Build>, shuttle_service::Error> {
    //          ^^^^^^^^^^^^^^^^^^^
    //                   |
    //                   This is the part to add

    // Business as usual here
}

[bmoxb]

All looks cool to me, the #[shared::Postgres], etc. proc-macro annotations are very funky :)