rgw: use unique_ptr for flat_map emplace in BucketTrimWatcher

rgw: use unique_ptr for flat_map emplace in BucketTrimWatcher

When emplacing objects into the trim notify handler of
BucketTrimWatcher, use a unique_ptr for the handler so that it is
destroyed if the emplace fails.

Though the destructor is already called, this behaviour cannot be relied
upon. std::map does not exhibit the same behaviour, and would have
leaked memory had it been used instead.

Fixes: https://tracker.ceph.com/issues/57938
Signed-off-by: Vedansh Bhartia <vedanshbhartia@gmail.com>