Investigate other ETL approaches

We should look at other ETL frameworks (both PHP and otherwise) and see what good ideas might be applicable to soong.

As a starting point, look at ETL packages on packagist (PHP) and the Awesome ETL list (non-PHP).

Let's treat this issue as a meta and open a separate issue for each library/framework/platform investigated.

Edit: On second thought let's make a list here of PHP ETL/migration packages on Packagist, and non-PHP frameworks. In most cases, a comment here and striking out the package/framework should suffice - only create a separate issue where a given package/framework has some particularly interesting ideas.

PHP packages

Stars/DLs	Package	Repo	Status
163/134859	portphp/portphp	https://github.com/portphp/portphp	See #15 (moved)
83/3819	knplabs/etl	https://github.com/docteurklein/php-etl
71/632	tdt/core	https://github.com/tdt/core
60/7398	marquine/php-etl	https://github.com/leomarquine/php-etl
29/261	umpirsky/extraload	https://github.com/umpirsky/Extraload
19/2605	bentools/etl	https://github.com/bpolaszek/bentools-etl
12/1161	bisight/etl	https://github.com/bisight/etl
8/4781	cleverage/process-bundle	https://github.com/cleverage/process-bundle
6/9136	fab2s/yaetl	https://github.com/fab2s/YaEtl
4/1381	tdt/input	https://github.com/tdt/input
3/3543	bookeen/etl-workflow	https://github.com/BOOKEEN/etl-workflow
2/57290	giift/etl	https://github.com/giift/etl
1/20	iu-redcap/redcap-etl	https://github.com/IUREDCap/redcap-etl
0/282	milesasylum/slurp	https://github.com/courtney-miles/slurp
0/166	yevgen-grytsay/etl-suite	https://github.com/yevgen-grytsay/etl-suite
0/121	xamplifier/etl	https://github.com/xamplifier/etl
0/117	cyve/etl	https://github.com/cyve/etl
0/98	inl/etl-processor	https://github.com/mpierzchalski/etl-processor
0/95	smartbooster/etl-bundle	https://github.com/smartbooster/etl-bundle
0/44	kiboko/etl	https://github.com/kiboko-labs/etl
0/33	ralphschindler/etl-pipeliner	https://github.com/ralphschindler/php-etl-pipeliner

Note the above come from searching packagist.org for "etl" (plus PortPHP suggested by @weitzman in https://gitlab.com/soongetl/soong/issues/8 - I've also searched on "migration", but at least among the first couple of pages of results they look pretty much to be about database migrations (e.g., changing schemas between different versions of a framework) and not applicable to this project.