Enable setting dbt_valid_to snapshot column to new setting dbt_valid_to_current

[gshank]

resolves #151

Problem

Support work being done in dbt-core and dbt-adapters to enable setting the value of a current row in the dbt_valid_to column.

Support dbt-core https://github.com/dbt-core/issue/10187, dbt-adapters dbt-labs/dbt-adapters#321.

Solution

Use the "dbt_valid_to_current" config.

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, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX