add sqlserver__alter_relation_add_remove_columns()

[sergey-vdovin]

missing implementation for alter_relation_add_remove_columns macro in 0.21.0rc1

https://github.com/dbt-labs/dbt-core/blob/8442fb66a591c4f5353f8baaaaf62f846c5f5171/core/dbt/include/global_project/macros/adapters/columns.sql

leads to incompatible with t-sql default statement generation "alter table xxx add column" when trying to use "append_new_columns" option for incremental models

[dataders]

thanks for identifying! can you tell me how this macro needs to be changed? or would be willing to open a pull request for this?

[sergey-vdovin]

at first glance looks as simple as removing "column" from "add column" but needs to be tested carefully: the second point in our discussion on slack was a bit more spooky:

https://getdbt.slack.com/archives/CMRMDDQ9W/p1636568890048600?thread_ts=1636568890.048600&cid=CMRMDDQ9W
for 0.21:
apparently there are some problems with "append_new_columns" https://docs.getdbt.com/docs/building-a-dbt-project/building-models/configuring-incremental-models

1. "alter table add column" is not valid in t-sql, "column" keyword should be remved
2. some columns from other models are trying to get into the model - some problems with metadata collection - in the same statement as for the problem 1