Snowpipe creation missing features from external tables

### Describe the feature
Snowflake external tables support the column alias and expression attributes,
but this only works when setting up external tables, the get_copy_sql macro for creating snowpipes doesn't support this. This can cause issues when raw data needs transforms that can't be done by simple type casting (eg nanosecond timestamps need a `to_timestamp($1:col, 9)` call instead of a simple `::timestamp` cast).
Snowpipe creation should also support these properties

### Describe alternatives you've considered
Don't think there are any? the properties already exist in the yaml, they are just not supported for snowpipes

### Additional context
For snowpipe creation on snowflake only