Set AbstractEnlaveLoader::Copy to public
Set AbstractEnlaveLoader::Copy to public

Previously Copy was protected and only accessible to |EnclaveManager|.
With this change it is now public so that an |AbstractEnclaveLoader|
can by copied elsewhere.

PiperOrigin-RevId: 262453461
Change-Id: If56bf3d6f988f2b14cbd9544410ca0753dd411a0

google/asylo,google/asylo,google/asylo,google/asylo,google/asylo,google/asylo