OvmfPkg: Introduce the OvmfSevMemoryAcceptance protocol

OvmfPkg: Introduce the OvmfSevMemoryAcceptance protocol

The default behavior for unaccepted memory in SEV-SNP is to accept all
memory when ExitBootServices is called. An OS loader can use this
protocol to disable this behavior to assume responsibility for memory
acceptance and to affirm that the OS can handle the unaccepted memory
type.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: "Min M. Xu" <min.m.xu@intel.com>
Cc: Andrew Fish <afish@apple.com>
Cc: "Michael D. Kinney" <michael.d.kinney@intel.com>
Signed-off-by: Dionna Glaze <dionnaglaze@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>