cmd: refactor exits

[KaloyanTanev]

Refactor exits logic. There is no change in the logic anywhere. The main differences are:

• fetching the validator index and validator pub key are in separate functions, which makes it more readable what is the flow
• ExpertMode is renamed to SkipBeaconNodeCheck as this is essentially what it does

If others agree, I'm in favour of completely removing the SkipBeaconNodeCheck config, as it is unnecessary.

category: refactor
ticket: none

[codecov]

Codecov Report

Attention: Patch coverage is 76.38889% with 17 lines in your changes missing coverage. Please review.

Project coverage is 57.64%. Comparing base (92ce181) to head (5b6e22f).

Files with missing lines	Patch %	Lines
cmd/exit_sign.go	76.38%	12 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3248      +/-   ##
==========================================
- Coverage   57.67%   57.64%   -0.03%     
==========================================
  Files         211      211              
  Lines       30372    30382      +10     
==========================================
- Hits        17517    17514       -3     
- Misses      10971    10980       +9     
- Partials     1884     1888       +4     

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[gsora]

Generally I like the idea, left a few comments that need clarification.

[sonarcloud]

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 SonarCloud