MdeModulePkg/Variable: Consolidate common parsing functions

MdeModulePkg/Variable: Consolidate common parsing functions

This change moves the following functions into a dedicated file
so they may be used in other variable files as needed. These are
commonly needed for basic variable data structure parsing
operations. The functions are grouped together in VariableParsing.c
to support cohesiveness for these operations in the file.
Furthermore, it reduces the overall size of the common Variable.c
file.

 * DataSizeOfVariable ()
 * FindVariableEx ()
 * GetEndPointer ()
 * GetNextVariablePtr ()
 * GetStartPointer ()
 * GetVariableDataOffset ()
 * GetVariableDataPtr ()
 * GetVariableHeaderSize ()
 * GetVariableNamePtr ()
 * GetVariableStoreStatus ()
 * GetVendorGuidPtr ()
 * IsValidVariableHeader ()
 * NameSizeOfVariable ()
 * SetDataSizeOfVariable ()
 * SetNameSizeOfVariable ()
 * UpdateVariableInfo ()
 * VariableCompareTimeStampInternal ()
 * VariableServiceGetNextVariableInternal ()

Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>