livepeer_cli: add menu option to exit gracefully

[emranemran]

What does this pull request do? Explain your changes. (required)
Adding new menu option to exit livepeer_cli gracefully without having to use Ctrl+C.

Specific updates (required)

• New menu option

How did you test each of these updates (required)
Tested locally and error code was verified. Attempted using other menu options prior to using the new exit option and everything worked as expected.

Does this pull request close any open issues?
fix #2287

Checklist:

• Read the contribution guide
• make runs successfully
• [ ] All tests in ./test.sh pass --> MacM1 fails locally on unrelated error but GH actions will pass
• README and other documentation updated
• Pending changelog updated

[emranemran]

Update: fixed go fmt build failure with tabs vs spaces.

[codecov]

Codecov Report

Merging #2438 (81d7dd4) into master (9f52b93) will decrease coverage by 0.04220%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##              master       #2438         +/-   ##
===================================================
- Coverage   55.00077%   54.95857%   -0.04221%     
===================================================
  Files             93          93                 
  Lines          19427       19431          +4     
===================================================
- Hits           10685       10679          -6     
- Misses          8150        8159          +9     
- Partials         592         593          +1     

Impacted Files	Coverage Δ
cmd/livepeer_cli/livepeer_cli.go	0.00000% <0.00000%> (ø)
server/mediaserver.go	66.71875% <0.00000%> (-0.46875%)	⬇️

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