support for virtual columns

[michalfairfly]

Hello, i have created for my internal needs support for virtual columns,
it could be useful for having access to simple automated calculations without need to store the column or remember the logic behind them (or creating a view):
Datetime columns in different timezones, for example arrival, arrival_in_utc, arrival_in_local_time_zone, arrival_in_departure_timezone

this type of construct will not affect incremental runs.

in file create.sql add following

    {%- set virtual_columns = config.get('virtual_columns', default=none) -%}

...
 -- add before clustering logic to allow for clustering over virtual column 
  {% if virtual_columns is not none %}
        {% for key, value in virtual_columns.items() %}
          alter table {{ relation }} add column {{ key }} {{ value.type }} as ({{ value.expression }});
        {% endfor %}
      {% endif %}

in impl.py
add to the SnowflakeConfig

@dataclass
class SnowflakeConfig(AdapterConfig):
   transient: Optional[bool] = None
   cluster_by: Optional[Union[str, List[str]]] = None
   automatic_clustering: Optional[bool] = None
   secure: Optional[bool] = None
   copy_grants: Optional[bool] = None
   snowflake_warehouse: Optional[str] = None
   query_tag: Optional[str] = None
   tmp_relation_type: Optional[str] = None
   merge_update_columns: Optional[str] = None
   target_lag: Optional[str] = None
   virtual_columns: Optional[Dict[str, Any]] = None

and then the config for snowflake model could look like as following:

{{
    config(
        materialized='incremental',
        unique_key='incremental_id',
        virtual_columns={
            'some_virtual_field': {"type": "float", "expression": "price* rate"},
            'another_virtual_field': {"type": "number(38,0)", "expression": "id + 1"}
        }
    )
}}

[dataders]

am I way off base in wanting to draw connections to both dbt-labs/dbt-external-tables#140 and dbt-labs/dbt-core#6929? In some sense a virtual column is a column alias that's kind of like a view materialization?