app/eth2wrap: fallback beacon nodes

[gsora]

Implement a way to provide eth2wrap with two classes of beacon nodes addresses: standard and fallback beacon nodes.

When one of the multi BN calls fails, eth2wrap wrappers will try to get an available fallback BN from a list and re-do the call on that.

If no fallback BNs is specified, return the original error.

If the fallback BN call fails, return fallback error instead.

This PR firstly introduces concepts and code, will introduce CLI parameters and initialization code later.

category: feature
ticket: #3328

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 31.86992% with 419 lines in your changes missing coverage. Please review.

Project coverage is 58.27%. Comparing base (4bbb36e) to head (1138ab3).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
app/eth2wrap/eth2wrap_gen.go	15.58%	388 Missing and 2 partials ⚠️
app/eth2wrap/multi.go	78.82%	18 Missing ⚠️
app/eth2wrap/eth2wrap.go	75.55%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3342      +/-   ##
==========================================
- Coverage   58.77%   58.27%   -0.51%     
==========================================
  Files         210      211       +1     
  Lines       30213    30699     +486     
==========================================
+ Hits        17758    17889     +131     
- Misses      10564    10915     +351     
- Partials     1891     1895       +4     

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