Add an API for paginated package versions

[brichet]

This PR add an API to retrieve paginated package versions, and a basic test on it.

[codecov-commenter]

Codecov Report

Merging #556 (fe51c24) into main (bee4a48) will decrease coverage by 0.79%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #556      +/-   ##
==========================================
- Coverage   82.87%   82.07%   -0.80%     
==========================================
  Files          78       78              
  Lines        5973     5987      +14     
==========================================
- Hits         4950     4914      -36     
- Misses       1023     1073      +50     

Impacted Files	Coverage Δ
quetz/dao.py	91.12% <100.00%> (+0.03%)	⬆️
quetz/main.py	86.85% <100.00%> (+0.65%)	⬆️
quetz/pkgstores.py	42.45% <0.00%> (-11.60%)	⬇️

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[hbcarlos]

Thanks, @brichet!