MdePkg/BaseLib: Add safe string functions [U|A]StrnTo[A|U]StrS

MdePkg/BaseLib: Add safe string functions [U|A]StrnTo[A|U]StrS

Add the following 2 APIs:
UnicodeStrnToAsciiStrS
AsciiStrnToUnicodeStrS

These APIs are used to enhance APIs UnicodeStrToAsciiStrS and
AsciiStrToUnicodeStrS (without 'n' in names) by:
1. Adds an input parameter 'Length' to specify the maximum number of
Ascii/Unicode characters to convert.
2. Adds an output parameter 'DestinationLength' to indicate the number of
Ascii/Unicode characters successfully converted.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>