Make YAML export of queries multiline

[shangyian]

Summary

This fixes the YAML export of nodes to have queries that are multiline rather than a single string separated by newlines.

The original code was modifying the yaml.SafeDumper class directly, which didn't reliably apply the multiline representer. The fix creates a new subclass each time. Now queries with newlines will be exported as:

  query: |
    select
    *
    FROM ...

Instead of:

  query: "select\n*\nFROM ..."

Test Plan

• PR has an associated issue: YAML export should use multi-line for queries #1661
• make check passes
• make test shows 100% unit test coverage

Deployment Plan

N/A

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