datasets: implement a fallback URL mechanism.

[ilia-kats]

This is apparently being used in the wild with workarounds, so we should provide it here.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.77%. Comparing base (de3484b) to head (de8a6ee).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/scverse_misc/datasets/_fetcher.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #75 +/- ##
==========================================
+ Coverage 92.63% 92.77% +0.13% 
==========================================
Files 11 11 Lines 584 609 +25 ==========================================
+ Hits 541 565 +24 - Misses 43 44 +1 

Files with missing lines	Coverage Δ
src/scverse_misc/datasets/_registry.py	100.00% <100.00%> (ø)
src/scverse_misc/datasets/_fetcher.py	98.24% <93.75%> (-1.76%)	⬇️

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