NetworkPkg: Add RAM disk boot support to HTTP Boot driver.

NetworkPkg: Add RAM disk boot support to HTTP Boot driver.

This patch updates the HTTP Boot driver to support the download and boot
a RAM disk image from HTTP server.
The HTTP RAM disk boot is described in section 23.7 "HTTP Boot" in
UEFI 2.6. HTTP server could provide either an UEFI image or a RAM disk image
for the HTTP boot client to use. The RAM disk image must contain a UEFI
compliant file system in it.
HTTP boot driver will identify the image type either by the "Content-Type"
entity header filed or by the file name extension as below:
  "application/efi" or *.efi -> EFI Image
  *.iso                      -> CD/DVD Image
  *.img                      -> Virtual Disk Image

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: El-Haj-Mahmoud Samer <samer.el-haj-mahmoud@hpe.com>