[BUG] Propagate URL download expressions max_connections to S3Config

[jaychia]

Problem: URL downloads had a regression when the default value on S3Config changed from 64 to 8.

This is because our URL download's max_connections parameter acts as a "throttling" mechanism for the maximum number of tasks in-flight from a channel buffer.

However, if this number is higher than the S3 client's max_connections, then it has no effect since the total number of connections to S3 is capped at S3Config.max_connections * num_threads.

In this PR, we just override the S3Config's value with the one that is passed into the expression.

[codecov]

Codecov Report

Merging #1708 (4212d25) into main (a53cd51) will decrease coverage by 0.23%.
Report is 4 commits behind head on main.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1708      +/-   ##
==========================================
- Coverage   85.11%   84.88%   -0.23%     
==========================================
  Files          55       55              
  Lines        5368     5393      +25     
==========================================
+ Hits         4569     4578       +9     
- Misses        799      815      +16     

Files	Coverage Δ
daft/expressions/expressions.py	92.09% <100.00%> (+0.04%)	⬆️

... and 10 files with indirect coverage changes