MdePkg: Add EFI_EVENT_BEFORE_EXIT_BOOT_SERVICES_GUID

MdePkg: Add EFI_EVENT_BEFORE_EXIT_BOOT_SERVICES_GUID

Event group as defined in UEFI standard v2.9.

Cc: Ard Biescheuvel <ardb@kernel.org>
Cc: "Min M. Xu" <min.m.xu@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Tom Lendacky <Thomas.Lendacky@amd.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
Message-Id: <20221108164616.3251967-3-dionnaglaze@google.com>