Lazily metadata loading

[wxicu]

535: update cell line metadata to 23Q4

478: Download the metadata upon specific request

PR Checklist

• [x] Referenced issue is linked
• [x] 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[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/theislab/pertpy/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab) | Coverage Δ | | |---|---|---| | [pertpy/metadata/\_look\_up.py](https://app.codecov.io/gh/theislab/pertpy/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L21ldGFkYXRhL19sb29rX3VwLnB5) | `10.45% <0.00%> (ø)` | | | [pertpy/metadata/\_drug.py](https://app.codecov.io/gh/theislab/pertpy/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L21ldGFkYXRhL19kcnVnLnB5) | `87.50% <66.66%> (-3.68%)` | :arrow_down: | | [pertpy/metadata/\_moa.py](https://app.codecov.io/gh/theislab/pertpy/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L21ldGFkYXRhL19tb2EucHk=) | `77.77% <66.66%> (-1.71%)` | :arrow_down: | | [pertpy/metadata/\_cell\_line.py](https://app.codecov.io/gh/theislab/pertpy/pull/544?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=theislab#diff-cGVydHB5L21ldGFkYXRhL19jZWxsX2xpbmUucHk=) | `51.17% <45.23%> (-7.75%)` | :arrow_down: |