engine: Add ExportToSst method with all logic moved to C++

Export was previously executed using an MVCCIncrementalIterator
where the logic handling iteration over the diff of the key
range [startKey, endKey) and time range (startTime, endTime]
was written in Go. This iteration involved making a cgo call
to find every key, along with another cgo call for writing
each key to the sstable.
This migration resolves the aforementioned performance
bottleneck (cockroachdb#18884), by pushing all the required logic to C++
and exposing a single export method.

Release note: Performance improvement to the BACKUP process.