PartitionCowCreator accounts for extra extents

dm-verity and error correction use part of the partitions using these
features to store their data.  Their data may be modified during an
update, so the COW device for the dm-snapshot must reserve some extra
space for them.
This patch extends the PartitionCowCreator data structure with the
extra_extents field that will store the (optional) additional extents,
e.g., the hash table extent for dm-verity or the error correction
extent.

Test: incremental OTA apply
Test: libsnapshot_test
Bug: 145180464
Change-Id: I387a6cc8438507ad41a85cc3400241ecaf627b8f
Signed-off-by: Alessio Balsini <balsini@google.com>