MdePkg/UefiLib: introduce EfiEventGroupSignal

MdePkg/UefiLib: introduce EfiEventGroupSignal

This is a small convenience function that eases signaling an event group
(identified by GUID). An example where it can be used is Platform BDS
signaling the End-of-DXE event group.

The naming follows EfiNamedEventSignal().

The patch modifies the library class header, and updates the most commonly
used library instance at once. Other library instances in the edk2 tree
will be adapted in the following patches.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>