Initial support for PostgreSQL

[bitfl0wer]

Initial support for using chorus with a PostgreSQL database, using the backend feature. In the same step, this removes support for MySQL/MariaDB. This does not mean that all types are now fully compatible with Postgres. Only the types needed in the current revision of symfonia have been ported over.

Interestingly, the sqlx docs mention the following when deriving sqlx::Type for structs in Postgres:

Records

User-defined composite types are supported through deriving a struct.

This is only supported for PostgreSQL. ⓘ

#[derive(sqlx::Type)]
#[sqlx(type_name = "interface_type")]
struct InterfaceType {
    name: String,
    supplier_id: i32,
    price: f64
}