Make 'x-ms-client-flatten' in typespec-azure-resource-manager templates conditional

[markcowl]

This requires updating the dependencies so that typespec-autorest depends on typespec-azure-resource-manager

• Add a new decorator @conditionaFlattening which marks a resource as flattened, depending on emitter settings. and an accessor isConditionallyFlattened, both of these apply to model properties
• Replace all template instances using @OpenApi.extension("x-ms-client-flatten": true) with the @conditionalFlattening` decorator.
• Add a new option in typespec-autorest arm-resource-flattening with values true and false, defaulting to false. If this option is true, use the isConditionallyFlattened accessor to determine whether to add an x-ms-client-flatten: true to the property representation in OpenAPI
• Update azure-rest-api-specs, with the new flag set to true in ARM projects