rest_dbus: implement route 'download/dump/<dumpid>
rest_dbus: implement route 'download/dump/<dumpid>

This allows user to download dump file from BMC using dump id.

'curl -O -J -c cjar -b cjar -k -X GET https://<bmc_IP>/download/dump/1'
- This will download the dump file on to the Host machine.

Resolves openbmc/openbmc#1508

Change-Id: I2821bfe8d68f80340512161c062503b92a7a7466
Signed-off-by: Jayanth Othayoth <d8c0d7e9f313f85e22ced997a3958fcdeb1cb606@in.ibm.com>

openbmc/phosphor-rest-server,openbmc/phosphor-rest-server