feat: support lists from form, query and file request inputs

[summersz]

implement a helper function to transform multidict into dict with lists
accept and return single values as a list if a list type is expected

closes #23

[Ge0rg3]

Hey @summersz thanks for the PR!
This currently does raise one issue:
The validator:

nicknames: List[str] = Json()

Should only explicitly allow lists, but under this implementation the value would be autoconverted, which ideally should only occur for input types without explicit list implementations. Let me make a couple changes to resolve this and we should be good to go 🙌