Implement `IntoFuture` trait in services

[ctaggart]

Add support like microsoft/azure-devops-rust-api#89. When complete, it should be possible to remove into_future() from:

azure-sdk-for-rust/services/mgmt/compute/examples/list_free_linux_images.rs

Lines 25 to 32 in 356952d

	let mut publishers = client
	.list_publishers(&location, &subscription_id)
	.into_future()
	.await?
	.into_iter()
	.map(|x| x.name)
	.collect::<Vec<_>>();
	publishers.sort();

If you try to remove it right now, you get:

PS C:\Users\cataggar\io\azure-sdk-for-rust\services\mgmt\compute> cargo check --examples
    Checking azure_mgmt_compute v0.9.0 (C:\Users\cataggar\io\azure-sdk-for-rust\services\mgmt\compute)
error[E0277]: `azure_mgmt_compute::virtual_machine_images::list_publishers::RequestBuilder` is not a future
  --> mgmt\compute\examples\list_free_linux_images.rs:26:54
   |
26 |           .list_publishers(&location, &subscription_id)
   |  ______________________________________________________^
27 | |         .await?
   | |______________^ `azure_mgmt_compute::virtual_machine_images::list_publishers::RequestBuilder` is not a future
   |
   = help: the trait `Future` is not implemented for `azure_mgmt_compute::virtual_machine_images::list_publishers::RequestBuilder`
   = note: azure_mgmt_compute::virtual_machine_images::list_publishers::RequestBuilder must be a future or must implement `IntoFuture` to be awaited
   = note: required because of the requirements on the impl of `IntoFuture` for `azure_mgmt_compute::virtual_machine_images::list_publishers::RequestBuilder`
help: remove the `.await`
   |
26 -         .list_publishers(&location, &subscription_id) 
26 +         .list_publishers(&location, &subscription_id)?
   |

For more information about this error, try `rustc --explain E0277`.
error: could not compile `azure_mgmt_compute` due to previous error