Add custom CI job exit code functions into scripts/utils.sh

What does this MR do and why?

Part 1 of !177976 (closed)

I decided to submit multiple MRs for the custom exit code change, starting with just the script itself and docs. I will not be using this script in any job yet, instead will make separate CI MRs to switch jobs to use the script individually. This way we can also easily revert only the CI changes if anything goes wrong.

re: Map CI/CD failures to failure categories (#512594 - closed)

References

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• first draft: Draft: Add a script file for custom exit code (!177976 - closed)
• issue: Map CI/CD failures to failure categories (#512594 - closed)

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Before	After

How to set up and validate locally

I ran 2 tests for the script in CI jobs:

Scenario 1: when job exits with 0

Job: https://gitlab.com/gitlab-org/gitlab/-/jobs/9020970173

This job runs with custom exit code, and the script passes. We confirm that the exit code 0 is correctly returned ✅

Scenario 2: when job exits with 1, while logging indicates gitaly spawn failure

Job: https://gitlab.com/gitlab-org/gitlab/-/jobs/9021120615

This job runs with custom exit code, and the job's command was modified to print "gitaly spawn failed" and exit with error. We confirm that the exit code 162 is correctly returned at the end of the job ✅