computed columns in external tables

Computed columns are supported on external tables in SQL, but are not supported on external tables in SQL projects.

https://developercommunity.visualstudio.com/t/Problem-with-adding-external-table-with-/10580514

https://learn.microsoft.com/en-us/answers/questions/1526682/how-to-add-external-table-with-computed-column-in

https://stackoverflow.com/questions/43365947/calculated-columns-in-sql-server-external-table

Computed columns seem to be supported on external tables across SQL Server, Azure SQL, and DW: https://learn.microsoft.com/en-us/sql/t-sql/statements/create-external-table-transact-sql

No source file, test, or entry point is named. Start by reproducing the external-table computed-column case in a SQL project and compare it with the supported SQL syntax described in the linked documentation. Done means SQL projects accept and preserve computed columns on external tables.

Written by the indexing model from the issue text.