Allow creation of snowpipes without full refresh

[mirkoprescha]

In case you have a large warehouse you don't want to import all existing files into the snowpipe table. Only after using --vars "ext_full_refresh: true" all the files linked to the snowflake stage will be imported. Each dbt run-operation stage_external_sources will create snowpipe(s) with create or replace. Currently, we can not use snowpipes with DBT on large s3 folders because dbt creates only a snowpipe when the table does not exist or with the applying --vars "ext_full_refresh: true". However, in both cases happens always a full copy of existing files.

Description & motivation

Checklist

• [x] 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)

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[github-actions[bot]]

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