Implement cancellation from the GUI

[iinuwa]

Right now, there's no way to "go back" in the GUI after selecting a transport. I'm not particularly concerned about the GUI, since that will likely be fully implemented in another project. However, we should add the ability to cancel an operation.

There should be a distinction between cancelling an operation for a particular device or transport, and cancelling the flow altogether. The device/transport cancellation should be used when the user selects some future "Go Back" button. There should be a separate button to bail on the WebAuthn flow altogether. Cancelling the the whole flow should also cancel the pending device/transport operations.

Whatever GUI implements this can decide whether they want to use the granular device cancellation or the nuclear WebAuthn flow cancellation.

[iinuwa]

Some more notes:

• to avoid malicious clients being able to deny service to the WebAuthn API, new requests should be rejected if there is an in-flight request already running.
• If a flow is cancelled, there should be some clear method. This will depend on the UI method (if the user clicks cancelled, perhaps it's not important to show cancel confirmation, but if it's cancelled due to timeout, or the original client aborted the thing, then the user should probably be notified.)

[iinuwa]

Scoped this issue to cancellation from the UI, which has been implemented in #68.

We should follow up on cancelling from the client in #90.

Closing.