Lazily metadata loading

[wxicu]

#535: update cell line metadata to 23Q4
#478: Download the metadata upon specific request
PR Checklist

• Referenced issue is linked
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Technical details

Additional context

[codecov]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (48ccd7f) 63.85% compared to head (f49175c) 63.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
- Coverage   63.85%   63.45%   -0.41%     
==========================================
  Files          43       43              
  Lines        5019     5046      +27     
==========================================
- Hits         3205     3202       -3     
- Misses       1814     1844      +30     

Files	Coverage Δ
pertpy/metadata/_look_up.py	10.45% <0.00%> (ø)
pertpy/metadata/_drug.py	87.50% <66.66%> (-3.68%)	⬇️
pertpy/metadata/_moa.py	77.77% <66.66%> (-1.71%)	⬇️
pertpy/metadata/_cell_line.py	51.17% <45.23%> (-7.75%)	⬇️