Add split() interpolation function for CI/CD inputs

What does this MR do and why?

Add split() interpolation function for CI/CD inputs

Introduces a new split('<separator>') function to the CI/CD interpolation stack (alongside truncate, expand_vars, posix_escape). The function splits a string input on a separator, strips whitespace from each element, and removes empty segments. Gated behind the ci_interpolation_split_function beta feature flag (disabled by default).

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

How to reproduce

1. Enable the feature flag:

   Feature.enable(:ci_interpolation_split_function)

2. Create a component or included CI file with a string input:

   # .gitlab/ci/deploy.yml
   spec:
     inputs:
       runner_tags:
         default: 'docker, linux'
   ---
   
   deploy:
     tags: $[[ inputs.runner_tags | split(',') ]]
     script: echo "Deploying with tags..."

3. Include it in your .gitlab-ci.yml:

   include:
     - local: '.gitlab/ci/deploy.yml'
       inputs:
         runner_tags: 'docker, linux, arm64'

4. The tags field resolves to ['docker', 'linux', 'arm64'] (whitespace stripped, empty segments removed).

Error cases to verify:

• Using split() with a non-string input returns: invalid input type: split can only be used with string inputs
• Using an empty separator split('') returns: invalid argument: separator cannot be empty
• Chaining functions after split() (e.g., split(',') | truncate(0,1)) returns: split() must be the last function in a chain (it returns an array, not a string)

References

Rollout issue: #603990

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