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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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[sonarcloud[bot]]

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