Remove 'x-ms-client-flatten' in typespec-azure-resource-manager templates and make it conditional in autorest

[allenjzhang]

Closes #709

[azure-sdk]

All changed packages have been documented.

• :white_check_mark: @azure-tools/typespec-autorest
• :white_check_mark: @azure-tools/typespec-azure-resource-manager

Show changes

### `@azure-tools/typespec-autorest` - _breaking_ [✏️](https://github.com/allenjzhang/typespec-azure/edit/azhang_RemoveClientFlatten/.chronus/changes/azhang_RemoveClientFlatten-2024-6-1-16-58-12.md?pr=/Azure/typespec-azure/pull/1105) > `x-ms-client-flatten` extension on some of resource properties property is now configurable to be emitted by autorest emitter. Default is false which will skip emission of that extension. ### `@azure-tools/typespec-azure-resource-manager` - _breaking_ [✏️](https://github.com/allenjzhang/typespec-azure/edit/azhang_RemoveClientFlatten/.chronus/changes/azhang_RemoveClientFlatten-2024-6-1-16-58-12.md?pr=/Azure/typespec-azure/pull/1105) > `x-ms-client-flatten` extension on some of resource properties property is now configurable to be emitted by autorest emitter. Default is false which will skip emission of that extension.

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/typespec-azure/prs/1105/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/typespec-azure/prs/1105/