Snowflake: Add expression parameter to columns

[kyleburke-meq]

resolves: #140

Adds the ability for custom expressions and identifiers to external sources in snowflake.

Description & motivation

All credit goes to @ipear3 and his original PR: #209
I am just resolving a merge conflict and want to see the code implemented.

Checklist

• I have verified that these changes work locally
• I have updated the README.md (if applicable)
• I have added an integration test for my fix/feature (if applicable)

[cakkinep]

depending on the order of the merges, we will have to resolve conflicts with #257 , i am adding column alias functionality there.

[kyleburke-meq]

@dataders is there something preventing this PR from being merged?

[the-davidsn]

+1
This feature is pretty helpful - especially when trying to handle timestamps w/ parquet files :)

[kyleburke-meq]

@dataders I have updated the integration test so it is now passing. I also resolved the conflict with #257.

Would you kindly review this again?

[cakkinep]

@kyleburke-meq thanks for resolving the conflicts and adding the integration test for the changes.

[dataders]

thanks @kyleburke-meq (and @jpear3)!

[cakkinep]

I think this change introduced the bug - #315

[kyleburke-meq]

I'm not sure if this is a bug. left a reply in that issue

[kyleburke-meq]

Ah actually I think I see the issue now. We should be using column_quoted instead of column_alias here I think?:

dbt-external-tables/macros/plugins/snowflake/create_external_table.sql

Line 43 in 4d26c38

{%- set col_id = 'value:c' ~ loop.index if is_csv else 'value:' ~ column_alias -%}