Support split eval function

[ahkcs]

Summary

This PR implements the split eval function for PPL, enabling users to split strings into multivalue arrays based on a delimiter.

Examples

Basic split with semicolon

source=people | eval result = split('a;b;c', ';')

Result: ['a', 'b', 'c']

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Split into individual characters (empty delimiter)

source=people | eval result = split('abcd', '')

Result: ['a', 'b', 'c', 'd']

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Multi-character delimiter

source=people | eval result = split('name::value', '::')

Result: ['name', 'value']

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Split field value

source=people | eval words = split(employer, ' ')

Splits the employer field on spaces.

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Check List

• New functionality includes testing.
• New functionality has been documented.
• New functionality has javadoc added.
• New functionality has a user manual doc added.
• New PPL command checklist all confirmed.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff or -s.
• Public documentation issue/PR created.

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