Add CAST as timestamp_default_dtype to postgres__string_to_timestamp

[tkiehn]

Description

Add CAST as timestamp_default_dtype to postgres__string_to_timestamp.
Allows users to define their desired timestamp datatype

[remoteworkflow]

Link to workflow summary: https://github.com/ScalefreeCOM/datavault4dbt-ci-cd/actions/runs/10956556632

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RESULTS for Synapse:
✅ dbt-tests
✅ dbt-macro-tests

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RESULTS for Postgres:
✅ dbt-tests
✅ dbt-macro-tests

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RESULTS for BigQuery:
✅ dbt-tests
✅ dbt-macro-tests

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RESULTS for Redshift:
✅ dbt-tests
✅ dbt-macro-tests

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RESULTS for Snowflake:
✅ dbt-tests
✅ dbt-macro-tests

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RESULTS for Exasol:
❌ dbt-tests
❌ dbt-macro-tests