Resolving the "select_as" issue

[mohs8421]

PR Info

This PR implements a test for the stated issue and aims to close it.

• Closes https://github.com/SeaQL/sea-orm/issues/1558

Bug Fixes

• fixes columns that have a "select_as" attribute in mysql not being correctly mapped

[mohs8421]

Hey @mohs8421, thanks for the PR!! I just added a commit to optimize the implementation where we will be able to correctly apply column alias for <SimpleExpr> AS <Alias> (SimpleExpr::Binary expression) when constructing "select_also" and "select_with" 8777788

sounds reasonable. I honestly saw this as an opportunity to prepare for this request: https://github.com/SeaQL/sea-orm/issues/1600 because it would of course be simpler to replace items, if the alias is defined.