feat: Add a way to get an expr.ExprBuilder from a plan.Builder

[EpsilonPrime]

By creating anexpr.ExprBuilder directly from the plan.Builder the two can share the same extension registry.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.61%. Comparing base (9b453f5) to head (fada7eb).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   59.56%   59.61%   +0.05%     
==========================================
  Files          43       43              
  Lines        9316     9321       +5     
==========================================
+ Hits         5549     5557       +8     
+ Misses       3507     3505       -2     
+ Partials      260      259       -1     

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