Allow client callbacks to dynamically set the next URL

[ml-evs]

This is useful for dynamic querying, e.g., changing a filter based on the number of results, or skipping forward to later pages.

[codecov]

Codecov Report

Merging #1685 (ff663f9) into master (a8e28e1) will decrease coverage by 0.06%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #1685      +/-   ##
==========================================
- Coverage   90.92%   90.87%   -0.06%     
==========================================
  Files          74       74              
  Lines        4608     4612       +4     
==========================================
+ Hits         4190     4191       +1     
- Misses        418      421       +3     

Flag	Coverage Δ
project	90.87% <94.11%> (-0.06%)	⬇️
validator	90.76% <94.11%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
optimade/client/client.py	81.92% <88.88%> (+0.16%)	⬆️
...made/server/entry_collections/entry_collections.py	95.83% <100.00%> (-0.05%)	⬇️
optimade/server/routers/links.py	100.00% <100.00%> (ø)
optimade/server/routers/references.py	100.00% <100.00%> (ø)
optimade/server/routers/structures.py	100.00% <100.00%> (ø)
optimade/server/routers/utils.py	95.12% <100.00%> (-0.82%)	⬇️

... and 1 file with indirect coverage changes