feat(stresstest): allow setting custom TTL

[matt-codecov]

i needed to generate a bunch of test data in our sandbox so i could test our GCS/Bigtable COGS rollup queries and measure their performance. this change to the stresstest is how i did it.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.47%. Comparing base (0bc8c3f) to head (5c22f7a).

Files with missing lines	Patch %	Lines
stresstest/src/config.rs	0.00%	10 Missing ⚠️
stresstest/src/main.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #564      +/-   ##
==========================================
- Coverage   87.55%   87.47%   -0.08%     
==========================================
  Files          93       93              
  Lines       14969    14983      +14     
==========================================
+ Hits        13106    13107       +1     
- Misses       1863     1876      +13     

Components	Coverage Δ
Rust Backend	92.35% <ø> (+<0.01%)	⬆️
Rust Client	79.89% <ø> (ø)
Python Client	90.20% <ø> (ø)

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