Add support to create a non-existent project dir if output_path is specified

[rghv404]

Currently, when we call:

pipx run openapi-python-client generate --url `URL` --output-path `NON_EXISTENT_PATH` 

we get an error

It'd be great if the library automatically handled creating a nonexistent project directory, especially if a custom destination path is provided.

Adding an exist_ok=True here should suffice

Happy to work on a PR if this make sense