OvmfPkg: add library class BlobVerifierLib with null implementation

BlobVerifierLib will be used to verify blobs fetching them from QEMU's
firmware config (fw_cfg) in platforms that enable such verification.

The null implementation BlobVerifierLibNull treats all blobs as valid.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ashish Kalra <ashish.kalra@amd.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: Min Xu <min.m.xu@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3457
Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

OvmfPkg/OvmfPkg.dec

@@ -23,6 +23,9 @@
   ##  @libraryclass  Access bhyve's firmware control interface.
   BhyveFwCtlLib|Include/Library/BhyveFwCtlLib.h
 
+  ##  @libraryclass  Verify blobs read from the VMM
+  BlobVerifierLib|Include/Library/BlobVerifierLib.h
+
   ##  @libraryclass  Loads and boots a Linux kernel image
   #
   LoadLinuxLib|Include/Library/LoadLinuxLib.h