Add GetUsageSummary additionalProperties example

[charlie-zhang109]

Summary

• Adds GetUsageSummary_additional_properties.ts example showing how to combine the v2 getUsageSummaryAvailableFields call with the v1 getUsageSummary call
• Demonstrates iterating responseFields, dateFields, and dateOrgFields over additionalProperties at each nesting level (top-level, per-date, per-org)
• Useful for customers who need to read all usage fields programmatically without hardcoding field names

Test plan

• Verify file compiles (tsc --noEmit from repo root or the examples tsconfig)
• Run against a test org to confirm all three field lists iterate correctly

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