`header` and `cookie` params in `req_modify()` and `req_prepare()`

[jonthegeek]

In req_modify() and passed up to req_prepare():

Add a header argument that accepts a list, and adds them to the request using httr2::req_headers().

Add a cookie argument that also accepts a list, and adds them to the request using httr2::req_cookies_set() (very similar to how the query arg is handled with .req_query_flatten()).

Document header and cookie in R/aaa-shared_params.R and inherit them in the functions that use them.