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 89.11%. Comparing base (1fe9c1b) to head (7cbf9ee).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10780      +/-   ##
==========================================
+ Coverage   86.31%   89.11%   +2.79%     
==========================================
  Files         182      182              
  Lines       23297    23298       +1     
==========================================
+ Hits        20109    20761     +652     
+ Misses       3188     2537     -651     

Flag	Coverage Δ
integration	86.32% <100.00%> (+0.01%)	⬆️
unit	62.20% <100.00%> (?)

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Components	Coverage Δ
Unit Tests	62.20% <100.00%> (∅)
Integration Tests	86.32% <100.00%> (+0.01%)	⬆️

[peterallenwebb]

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