Add http_seekable option for HTTP partial requests, when The
EXT-X-BYTERANGE tag indicates that a Media Segment is a sub-range
of the resource identified by its URI, we can use HTTP partial
requests to get the Media Segment.
Reviewed-by: Steven Liu <lq@chinaffmpeg.org>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>