Cross-database date macro

[dbeatty10]

resolves #192

Problem

To support a built-in metricflow_time_spine in dbt-labs/dbt-core#8825, we want to create a date macro that converts (year, month, day) arguments into a SQL DATE type.

Solution

Add a new cross-database macro named date() that takes three integer parameters (year, month, day) and renders SQL that will create an associated scalar date value in the target data platform.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests
• This PR has already received feedback and approval from Product or DX