Enable setting datetime value for dbt_valid_to when the record is current

[gshank]

Resolves #10187

Problem

Currently we use a null to indicate that a snapshot record is current. Users would like to use some arbitrary future date to indicate current instead, because it makes some SQL easier.

Solution

Add a new attribute "dbt_valid_to_current", and implement changes in adapters to use it.

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 or 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.
• This PR includes type annotations for new and modified functions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.18%. Comparing base (f6cdacc) to head (5fc8a19).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10780      +/-   ##
==========================================
- Coverage   89.20%   89.18%   -0.03%     
==========================================
  Files         183      183              
  Lines       23419    23426       +7     
==========================================
+ Hits        20892    20893       +1     
- Misses       2527     2533       +6     

Flag	Coverage Δ
integration	86.50% <100.00%> (-0.01%)	⬇️
unit	62.11% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
Unit Tests	62.11% <100.00%> (+<0.01%)	⬆️
Integration Tests	86.50% <100.00%> (-0.01%)	⬇️

[peterallenwebb]

Reminder, we need to unpin the dependency branches before merge. Otherwise this looks good.