IVGCVSW-6073 Add protected mode to ArmNN CreationOptions

 * Adds logic to the Runtime to activate protected mode
 * Adds ProtectedContentAllocation backend capability to ClBackend
   It's not fully activated yet because the CustomAllocator is
   missing. Will print an error message and won't register the backend
   but won't fail.
 * Extends IBackendInternal with an UseCustomAllocator function.
 * Adds related unit tests

Signed-off-by: Jan Eilers <jan.eilers@arm.com>
Change-Id: I64f465c5800eb104aa90db1bbf772a4148b5072f