Deprecate per-request cookies

[lovelydinosaur]

Closes #1404

Documentation here is as follows...

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Cookies

If using a client instance, then cookies should always be set on the client rather than on a per-request basis.

This usage is supported...

client=httpx.Client(cookies=...)
client.post(...)

This usage is deprecated...

client=httpx.Client()
client.post(..., cookies=...)

We prefer enforcing a stricter API here because it provides clearer expectations around cookie persistence, particularly when redirects occur.

[rafalkrupinski]

I would really hate loosing per-request cookies. Cookies are just a header, why the Cookies facility class should be disabled when doing redirects?

Wouldn't it be reasonable to expect request cookies to follow the redirects, either by default or with a flag?
I'll follow up in #1404