OvmfPkg/BaseMemEncryptTdxLib: Add TDX helper library

RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3429

Add Intel Tdx helper library. The library provides the routines to:
-  set or clear Shared bit for a given memory region.
-  query whether TDX is enabled.

Please be noted, there a lot of duplicated codes for Page Table
operations. These codes should be moved to a common library
(PageTablesLib) so that it is more friendly for review and maintain.
There is a new feature requirement
https://bugzilla.tianocore.org/show_bug.cgi?id=847 which is to
implement the library. After the lib is introduced, BaseMemEncryptTdxLib
will be refactored.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

OvmfPkg/OvmfPkg.dec

@@ -34,6 +34,10 @@
   #                  Virtualization (SEV) guests.
   MemEncryptSevLib|Include/Library/MemEncryptSevLib.h
 
+  ##  @libraryclass  Declares helper functions for TDX guests.
+  #
+  MemEncryptTdxLib|Include/Library/MemEncryptTdxLib.h
+
   ##  @libraryclass  Save and restore variables using a file
   #
   NvVarsFileLib|Include/Library/NvVarsFileLib.h