Allow generation of subdirs when using '--template-dir'

[ThisIsANiceName]

Hi.

I'd love to be able to generate sub directories when using the --template-dir option. Currently this is not possible as in fastapi_code_generator/main for target in template_dir.rglob("*"): is used instead of for target in template_dir.rglob("*.jinja2"): and parsing a directory path to the code gen throws an error.

Furthermore it would be cool to automatically generate subpaths if they do not exist.

Example:

templates:
  generated:
    router.jinja2
  main.jinja2

should result in
main.py, generated/router.py

This would help me, structuring the generated code further.

I'd like to make a pull request for this, but would love some input first :)