Extract New-FinOpsExportManifest as a reusable PowerShell command

Summary

Extract the manifest generation logic from New-FinOpsTestData into a standalone New-FinOpsExportManifest command that can be used independently.

Motivation

New-FinOpsTestData currently generates Cost Management-compatible manifest files inline. This logic is useful beyond test data — anyone creating custom data pipelines or tooling that writes to FinOps hubs storage could benefit from a dedicated manifest command.

Proposed changes

• Create a new New-FinOpsExportManifest command in the PowerShell module
• Support all manifest fields (blobCount, blobs, byteCount, dataFormat, exportConfig, runInfo, etc.)
• Support the _ftkTestData watermark fields as optional parameters
• Refactor New-FinOpsTestData to call New-FinOpsExportManifest internally
• Add Pester tests and MS Learn documentation

Context

Deferred from PR #2006 (New-FinOpsTestData and Remove-FinOpsTestData).

Start by tracing the manifest generation logic in New-FinOpsTestData and identify every manifest field, including the optional _ftkTestData watermark fields. Extract it into the New-FinOpsExportManifest command, update New-FinOpsTestData to use it, then add Pester coverage and the corresponding MS Learn documentation.