UPDATE...RETURNING support

[DanilDelegator]

issue link: https://github.com/JetBrains/Exposed/issues/351

Known problems:

1. Should it be a different API updateReturning or should I pile it on the existing update somehow?
2. About 1/2 of the supported platforms do not support this feature. In the current implementation if you try to use it with an incompatible platform you'll get a runtime error. It seems like that is happening all over the project though so I'm not sure if it must be fixed for this PR.

[stengvac]

Hi,

Maybe you can use some pieces from https://github.com/JetBrains/Exposed/pull/1456/files I did try to support returning for postgres in there.

[bog-walk]

Hi @DanilDelegator Thanks for starting a draft for this feature request.

A bit of work will need to be done to lay the foundation for a solid integration of the RETURNING clause, before this PR (and later ones for insert & delete) is adjusted. I'm putting together a design plan now and testing some API changes in preparation.

Please let me know if you will be willing and able to work further on this PR in the near future. Otherwise, I'll make the relevant edits to this PR directly.

[bog-walk]

Attempting to rebase and resolve directly on this fork branch turned out messier than hoped given the project repo directory changes.

Closing in favour of PR #2062