[codegen] create the root folder if not exists

[billy1624]

Following command should output the source code to mysql folder in the current directory. However, if the directory doesn't exist, it will result in an error.

➜ cargo r mysql://root:root@localhost/sakila mysql mysql
    Finished dev [unoptimized + debuginfo] target(s) in 0.80s
     Running `target/debug/seaography 'mysql://root:root@localhost/sakila' mysql mysql`
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: IoError(Os { code: 2, kind: NotFound, message: "No such file or directory" })', src/main.rs:19:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I think we should always check the existence of the target directory before writing Cargo.toml file to it. Perhaps adding std::fs::create_dir_all(path.as_ref())?; before writer::write_cargo_toml(path, crate_name, &sql_version)?; at:

• seaography/generator/src/lib.rs

  Lines 80 to 85 in 28ae5ce

  	let (tables, sql_version) =
  	seaography_discoverer::extract_database_metadata(&database_url).await?;
  	writer::write_cargo_toml(path, crate_name, &sql_version)?;
  	std::fs::create_dir_all(&path.as_ref().join("src/entities"))?;