implement necessary postgres cross_dbt_utils macros

[JLDLaughlin]

For each of these macros, either confirm that the Postgres implementation is sufficient, or add a materialize__* implementation in this repository:

• [ ] datediff
• [x] dateadd
• [x] current_timestamp_in_utc
• [ ] last_day
• [x] type_string

[JLDLaughlin]

Closing in favor of https://github.com/MaterializeInc/materialize-dbt-utils/issues/7