func_curl.c: Support custom http headers

func_curl.c: Support custom http headers

When user wants to send json data, the default Content-Type header
is incorect (application/x-www-form-urlencoded). This patch allows
to set any custom headers so the Content-Type header can be
overriden. User can set multiple headers by multiple calls of
curlopt(). This approach is not consistent with other parameters,
but is more readable in dialplan than one call with multiple
headers.

ASTERISK-28613

Change-Id: I4dd68c3f4e25362ef941d73a3861f58348dcfbf9