*: check fee recipient on local

[KaloyanTanev]

Recently we had an issue where the prepare beacon proposer request at a BN was failing silently, because it was passing at another BN.

This PR changes the checks for the warning of mismatching fee recipient. Previously the check was triggered on MEV disabled, now it checks on block being local (which is the case when MEV fails for one reason or another).

I've added also a healthcheck for the fee recipient mismatching.

Mind you, this won't solve the issue, as it requires greater refactoring on how things work in go-eth2-client, but it will make it easier to spot such issues.

category: feature
ticket: none

[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 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.52%. Comparing base (d3c1027) to head (c6b1ac4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
app/health/checks.go	60.00%	1 Missing and 1 partial ⚠️
core/fetcher/fetcher.go	88.88%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4474      +/-   ##
==========================================
- Coverage   56.57%   56.52%   -0.06%     
==========================================
  Files         245      245              
  Lines       32826    32819       -7     
==========================================
- Hits        18572    18551      -21     
- Misses      11893    11906      +13     
- Partials     2361     2362       +1     

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