*: track indices for which duties were fetched in cache

[KaloyanTanev]

Save indices for which the request was done. This helps with proposal duties caching, as for a request of N indices only a subset (if any) are returned. However, if we do not store the indices for which the cache was created, we cannot know if the initial request was for all indices or only for 1.

category: bug
ticket: #4009

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 21.00840% with 94 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.34%. Comparing base (4070c56) to head (36e174c).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
app/eth2wrap/cache.go	20.33%	93 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4333      +/-   ##
==========================================
- Coverage   56.39%   56.34%   -0.06%     
==========================================
  Files         237      237              
  Lines       31660    31696      +36     
==========================================
+ Hits        17856    17858       +2     
- Misses      11517    11548      +31     
- Partials     2287     2290       +3     

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