OvmfPkg/EnrollDefaultKeys: extract typedefs to a header file

OvmfPkg/EnrollDefaultKeys: extract typedefs to a header file

"EnrollDefaultKeys.c" defines three structure types: SINGLE_HEADER,
REPEATING_HEADER, and SETTINGS. The definitions are scattered over the C
file, and lack high-level summary comments.

Extract the structures to "EnrollDefaultKeys.h", and add the missing
comments.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@arm.com>
Bugzilla: https://bugzilla.tianocore.org/show_bug.cgi?id=1747
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Gary Lin <glin@suse.com>