Add example: read UsageSummary fields via additionalProperties

[charlie-zhang109]

Summary

• Adds examples/v2/usage-metering/GetUsageSummary_additional_properties.java, a worked example showing how to enumerate all fields returned by GET /api/v1/usage/summary — including newly added billing dimensions that land in additionalProperties rather than typed fields.
• Calls GET /api/v2/usage/summary/available_fields (v2) first to retrieve the canonical field lists (responseFields, dateFields, dateOrgFields) from UsageSummaryAvailableFieldsAttributes.
• Calls GET /api/v1/usage/summary (v1) with includeOrgDetails(true) to get the full three-level hierarchy.
• Iterates all three levels — response, UsageSummaryDate, and UsageSummaryDateOrg — calling getAdditionalProperty(field) for each discovered key and printing non-null values.

Test plan

• Review the example compiles against the existing models in src/main/java/com/datadog/api/client/v1/model/ and src/main/java/com/datadog/api/client/v2/model/.
• Verify import paths: com.datadog.api.client.v1.model.{UsageSummaryResponse,UsageSummaryDate,UsageSummaryDateOrg} and com.datadog.api.client.v2.model.{UsageSummaryAvailableFieldsResponse,UsageSummaryAvailableFieldsAttributes}.
• Run manually against a real org to confirm all three levels print non-null values for known fields.

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