OvmfPkg: Add TdxMailboxLib

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

In Tdx BSP may issues commands to APs for some task, for example, to
accept pages paralelly. BSP also need to wait until all the APs have
done the task. TdxMailboxLib wraps these common funtions for BSP.

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

@@ -109,6 +109,10 @@
   #
   XenPlatformLib|Include/Library/XenPlatformLib.h
 
+  ##  @libraryclass  TdxMailboxLib
+  #
+  TdxMailboxLib|Include/Library/TdxMailboxLib.h
+
 [Guids]
   gUefiOvmfPkgTokenSpaceGuid            = {0x93bb96af, 0xb9f2, 0x4eb8, {0x94, 0x62, 0xe0, 0xba, 0x74, 0x56, 0x42, 0x36}}
   gEfiXenInfoGuid                       = {0xd3b46f3b, 0xd441, 0x1244, {0x9a, 0x12, 0x0, 0x12, 0x27, 0x3f, 0xc1, 0x4d}}