Add metadata_file_last_modified for snowpiped tables

[Catisyf]

Description & motivation

resolves: #276

dbt currently supports the following metadata columns for snowpiped table.

    #   `metadata_filename`: the file from which this row was loaded
    #   `metadata_file_row_number`: the numbered row this was in that file
    #   `_dbt_copied_at`: the current_timestamp when this row was loaded (backfilled or piped)

This PR adds a new metadata column metadata_file_last_modified (metadata$file_last_modified), which returns the last modified timestamp of the staged file the current row belongs to. This metadata is useful for a use case where we'd like to filter staged files from an S3 bucket after a certain cutoff date, and we can't easily infer such date from the file name.

This is a simple modification and I have tested the changes in our dbt project. It works as expected.

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)

[jtmcn]

Please do add this functionality. Although I'd suggest replacing the current _dbt_copied_at field which uses current_timestamp since Snowflake recommends against using that function in a Snowpipe.

https://community.snowflake.com/s/article/Incorrect-timestamp-in-Snowpipe-while-using-current-timestampfunction

https://docs.snowflake.com/en/user-guide/data-load-snowpipe-ts#load-times-inserted-using-current-timestamp-earlier-than-load-time-values-in-copy-history-view

[dataders]

@jtmcn see #281 for your suggestion. I'd welcome PR for this!

[dataders]

thanks @Catisyf!