Add AlterPath and Traverse functions to mapstr.M

[rdner]

What does this PR do?

This allows to make changes to the existing keys easier. For example, switching casing of the keys.

Why is it important?

It helps to implement 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

• Follow up to Add a function to perform case-insensitive search in mapstr.M #244
• Related to [libbeat]: Add lowercase processor beats#41424

[rdner]

The failed Sonar check is a false-positive, detected different test cases as duplicated lines.

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 748ef74

cc @rdner