Support a way to configure partitioning by segment size

[julienrf]

Currently, you can manually set scanSegments to control the number of partitions to split the data into.

We could also provide a segmentSize setting which could be an alternative to scanSegments to control the partitioning. The benefits would be that the same configuration (e.g. segmentSize: 100) could be used with different tables and it would automatically compute an appropriate number of segments that fit the desired size. Whereas with scanSegments you have to adapt its value to every table you migrate (because you want to adjust the number of segments according to each table size).

Originally posted by @julienrf in #143 (comment)