Some improvements

[AdiEcho]

1. get_cookies and set_cookies use domain instead of url to get cookies
2. Add argument follow_redirects for request function
3. The headers in the request method should not be ignored but should be merged with the default headers.

further

1. Add a cookies variable to AsyncClient, pointing to the internal cookies implementation. This variable can be used to retrieve all currently stored cookies, set a specific cookie value, or get a specific cookie value. You can refer to the implementation of other networking libraries for details.

[AdiEcho]

I noticed that the developer removed client.cookies in the commit history. Could you explain the reasoning behind this decision?