app: warn on proposer config builder mismatch

[KaloyanTanev]

Checks the builder settings of an existing vc-config/proposer-config.json on start against the builder flags, and warns on a mismatch. The flags only seed a new file; changing the builders of an existing one requires charon proposer-config edit, run in sync with the other peers. The file itself is never modified.

category: feature
ticket: #4324

[codecov]

Codecov Report

❌ Patch coverage is 69.69697% with 10 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (gloas@282a123). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
app/proposerconfig.go	69.69%	10 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             gloas    #4784   +/-   ##
========================================
  Coverage         ?   67.23%           
========================================
  Files            ?      252           
  Lines            ?    33030           
  Branches         ?        0           
========================================
  Hits             ?    22209           
  Misses           ?    10820           
  Partials         ?        1           

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[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

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[OisinKyne]

Doesn't actually have to be fixed before merge, just logging it.

One thing to fix before merge: the warning tells operators to run charon proposerconfig edit, but that command doesn't exist. The PR description also spells it differently (proposer-config edit). Until the command lands, operators who see the warning have nothing they can do. The fix that works today is to delete the file and restart, which regenerates it from the current flags and overrides. Suggested wording:

▎ Builder flags differ from the proposer config file, which is only generated when missing. Delete it and restart to regenerate it, in sync with the other peers.