[FEAT] Timestamp Truncation

[colin-ho]

Closes #2107

This PR adds a method to truncate the datetime column to the specified interval e.g. "1 day".

Valid time units are: 'microsecond', 'millisecond', 'second', 'minute', 'hour', 'day', 'week'.
Optional start time for truncation. If provided, truncation will be done from this start time, otherwise truncation will be done from the beginning of the epoch.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 85.08%. Comparing base (1e20b2d) to head (494bd37).
Report is 2 commits behind head on main.

❗ Current head 494bd37 differs from pull request most recent head e229c0d. Consider uploading reports for the commit e229c0d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2158      +/-   ##
==========================================
+ Coverage   85.06%   85.08%   +0.01%     
==========================================
  Files          68       68              
  Lines        7360     7376      +16     
==========================================
+ Hits         6261     6276      +15     
- Misses       1099     1100       +1     

Files	Coverage Δ
daft/expressions/expressions.py	92.82% <100.00%> (+0.04%)	⬆️
daft/series.py	93.11% <83.33%> (-0.12%)	⬇️

... and 3 files with indirect coverage changes

[jaychia]

Nice! Doing the math ourselves was a good call :)