Add a function to perform case-insensitive search in mapstr.M

[rdner]

What does this PR do?

FindFold (similar to strings.EqualFold) traverses the map and tries to perform a case-insensitive match of each key segment on each map level.

Why is it important?

It's a prerequisite for elastic/beats#41424

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Relates [libbeat]: Add lowercase processor beats#41424
• Relates Filebeat Processors - make uppercase and lowercase processors (as in ES ingest nodes) available to filebeat beats#22254

[elasticmachine]

💚 Build Succeeded

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• Commit: 48bc6f5

History

• 💚 Build #443 succeeded 8448d19

cc @rdner