[FEAT] [New Query Planner] Add support for `df.count_rows().

[clarkzinzow]

This PR adds support for df.count_rows(), the final missing implementation from the LogicalPlanBuilder interface for the new query planner.

[codecov]

Codecov Report

Merging #1273 (0039978) into main (a5c702b) will decrease coverage by 0.03%.
Report is 1 commits behind head on main.
The diff coverage is 70.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1273      +/-   ##
==========================================
- Coverage   87.75%   87.73%   -0.03%     
==========================================
  Files          61       61              
  Lines        6021     6023       +2     
==========================================
  Hits         5284     5284              
- Misses        737      739       +2     

Files Changed	Coverage Δ
daft/logical/rust_logical_plan.py	87.89% <25.00%> (-1.14%)	⬇️
daft/expressions/expressions.py	91.83% <100.00%> (ø)
daft/series.py	92.09% <100.00%> (ø)