Add SetCookie utility

[lundberg]

Adds a respx.SetCookie utility that can be used for easier mocking of response Set-Cookie headers.

The route.respond(...) helper is also enhanced with a cookies kwarg that supports dict[str, str], Sequence[tuple[str, str]] or Sequence[SetCookie].

Example usage via headers:

respx.post("https://example.com/").mock(
    return_value=httpx.Response(200, headers=[SetCookie("foo", "bar")])
)

.. or by using respond ..

respx.post("https://example.com/").respond(cookies={"foo": "bar"})
respx.post("https://example.com/").respond(cookies=[SetCookie("foo", "bar", path="/", ...)])

Fixes #249

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (24ee4a9) to head (c670ccb).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #254   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           21        22    +1     
  Lines         2845      2910   +65     
  Branches       428       443   +15     
=========================================
+ Hits          2845      2910   +65     

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