append HTTP headers with rest_client

[liviuchircu]

Follow-up of a discussion started with @jarrodb on IRC. In short, there are quite a lot of usage scenarios where additional HTTP headers need to be inserted into the request. So, the usage flow could be:

rest_append_hf("Authorization: Bearer mF_9.B5f-4.1JqM");
...
rest_get(...)

[jarrodb]

Also, there may have been the potential for a memory leak in start_async_http_req where (struct curl_slist *) list was allocated at rest_methods.c:130 and hit goto error; at rest_methods.c:183 which bypassed the curl_slist_free_all(list); in the success: label.

Pull request #497 adds the free call to the error: label as well.

[bogdan-iancu]

@jarrodb , thanks for your PR on this topic. @liviuchircu already accepted it, but could you please create a new PR updating the docs too (documenting the new script function) ?
Thanks and regards, Bogdan

[jarrodb]

Yessir I will work on documentation for the additional command.

[bogdan-iancu]

Thank you @jarrodb !