refactor: Improve table saver to always use the correct service worker

[mattrunyon]

Fixes #766

BREAKING CHANGE: TableSaver now expects the service worker to send it a complete URL for download instead of just a file name. DHE will need to adjust its serviceWorker.js to incorporate the same changes from this PR.

[codecov]

Codecov Report

Patch coverage: 9.09% and project coverage change: +0.65% 🎉

Comparison is base (848fef4) 45.77% compared to head (817cf30) 46.43%.
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1515      +/-   ##
==========================================
+ Coverage   45.77%   46.43%   +0.65%     
==========================================
  Files         518      558      +40     
  Lines       35177    35701     +524     
  Branches     8813     8912      +99     
==========================================
+ Hits        16103    16576     +473     
- Misses      19023    19075      +52     
+ Partials       51       50       -1     

Flag	Coverage Δ
unit	46.43% <9.09%> (+0.65%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
packages/iris-grid/src/sidebar/TableSaver.tsx	11.50% <0.00%> (-0.25%)	⬇️
...ages/code-studio/src/DownloadServiceWorkerUtils.ts	11.11% <16.66%> (+4.44%)	⬆️

... and 85 files with indirect coverage changes

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