Alternative layouts for SST files

@siying shared a post about HTAP. It reminded me of the Ressi data layout in [Spanner](https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/46103.pdf), which is based on [PAX](https://www.pdl.cmu.edu/PDL-FTP/Database/pax.pdf).

After moving from local storage to cloud storage, all SST files are stored in the cloud filesystem. Similar to the design of remote compaction contributed by @jay-zhuang, maybe the distributed processing frameworks like Spark can be used to convert the existing SST file formats to some alternative format like PAX for experiments.