Make client.cookies setter expect a Cookies instance

[florimondmanca]

Prompted by #677 (comment)

@tomchristie IMO the only thing that does look odd (at least from a typing perspective) is the setter magic, so this PR focuses on that. In particular, not sure we really want to get rid of CookieTypes and change the cookies parameters to prevent using the convenience dict/CookieJar types.

[florimondmanca]

@tomchristie Would we want to add an assert or any kind of runtime type check here? I'm not sure it's really needed.

Also, this change makes the cookies property equivalent to a simple variable, so we might as well switch it to just that — a plain and public .cookies variable?

[tomchristie]

I think we should probably only do this if we're also doing the same for Headers and Params.
It's very possibly a good thing to do in all cases, but perhaps let's defer that decision for now, and allow the type: ignore on https://github.com/encode/httpx/pull/677/files#diff-2aeaac4c23c1f6bf49dfbd99885a3aa7L108 for the moment.

[florimondmanca]

Closing this one for now to be addressed later with a pass on Headers and Params, then.