examples-migrate tool

[timotheeguerin]

Part of #4831.

RFC: https://github.com/Azure/azure-rest-api-specs/blob/rfc/unified-examples-format/documentation/rfc/unified-examples-format.md

Build examples-migrate: convert existing x-ms-examples JSON into examples.yaml.

Scope

• Read all x-ms-examples JSON for a service and emit a single examples.yaml (or examples/<Interface>.yaml for large services).
• Dedup across versions into since variants (full restatement, no partial overrides).
• Normalize embedded version strings to {api-version} in headers and body values (e.g. Location, Azure-AsyncOperation, nextLink) so trivially-versioned variants collapse.
• Emit interface-relative operation keys under a single $namespace.

Depends on #4832 (schema).