Enable setting current value of dbt_valid_to

[gshank]

resolves #320

Problem

To support setting the snapshot dbt_valid_to column to a date value instead of null. For dbt-core dbt-labs/dbt-core#10780.

Solution

Use the setting of the new attribute "dbt_valid_to_current" instead of (or in addition to) null in various snapshot queries.

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