net: lib: http: Added Content-Range header parsing.

Content-Range hasn't been supported in zephyr. This change adds
Content-Range header parsing to http_parser module. Range start,
range end, and total size are supported. All units are currently
interpreted as bytes.
This is much needed change, because many applications responsible
for http data download are based on Content-Range approach.

Signed-off-by: Piotr Radecki <piotr.radecki@jrdltd.co.uk>