NetworkPkg: Add dns support for pxe boot based on IPv6.

NetworkPkg: Add dns support for pxe boot based on IPv6.

The BootFileURL option (59) in dhcpv6 is used to deliver
the next server address with bootfile name, as an example
"tftp://[FEDC:BA98:7654:3210:FEDC:BA98:7654:3210]/BOOTFILE_NAME;
mode=octet", it can also be “tftp://domain_name/BOOTFILE_NAME;
mode=octet”, this patch is to support this case.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Cc: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>