feat: fetch all targets for all upstreams at once

[GGabriele]

Avoid needing to loop through all upstreams to fetch all targets.

Requires:

• feat: add methods to fetch all targets for all upstreams go-kong#599
• feat: fetch all targets for all upstreams at once go-database-reconciler#365

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 32.79%. Comparing base (19a389c) to head (fc17f2e).

Files with missing lines	Patch %	Lines
tests/integration/test_utils.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1845      +/-   ##
==========================================
- Coverage   32.84%   32.79%   -0.05%     
==========================================
  Files          73       73              
  Lines        6556     6556              
==========================================
- Hits         2153     2150       -3     
- Misses       4240     4242       +2     
- Partials      163      164       +1     

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