WIP: Add converters from typed-settings

[sscherfke]

See: #813

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[sscherfke]

Typing can be quite tricky. I’d be grateful if anyone could improve the type hints in converters.pyi. :-)

[Tinche]

Heh, I'm just finishing up a blog post about how attrs+cattrs is better than Pydantic since these kinds of type-altering converters don't belong on the models themselves. A small taste of the post: what if you want two code paths for converting datetimes, one for ISO 8601 and one for the unix timestamps? Also paying the price for those code paths in your code (i.e. when not loading from json or whatever) is silly.

Well, at least in attrs they would be opt-in if we merge this, instead of just there forever like in Pydantic (I'm aware of Pydantic ORM mode).

[sscherfke]

Yes, I see your point. But these converters (and maybe eventually the auto-convert hook) are purely optional and make writing simple API clients or loading data from JSON a lot easier. And you don't need to install the additional cattr packages for these simple cases.

[sscherfke]

Closing this in favor of #830