add "rt-multi-thread" as dev-dependency for services

[ctaggart]

This matches the SDKs and avoids this error:

error: The default runtime flavor is `multi_thread`, but the `rt-multi-thread` feature is disabled.
  --> mgmt/vmware/examples/private_cloud_list.rs:17:1
   |
17 | #[tokio::main]
   | ^^^^^^^^^^^^^^
   |
   = note: this error originates in the attribute macro `tokio::main` (in Nightly builds, run with -Z macro-backtrace for more info)

https://github.com/Azure/azure-sdk-for-rust/runs/7843927677?check_suite_focus=true