syz-manager: use a separate type for corpus items
syz-manager: use a separate type for corpus items

Now we use the same type for both RCP communication with syz-fuzzer and
for managing corpus. It's not convenient to add corpus-specific fields
in this case, so separate them.

xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller