Support cancellation in deployers

[jmrossy]

Problem

Once a deployment triggered (e.g. via await MyDeployer.deploy()), there is no good way to stop it. The Deploy App hacked cancellation support by bricking the multiProvider passed into the deployer but it's not super reliable. The proper solution would be to have the deployers check for cancellation requests in between read/write calls and bail if needed.

AbortController may help with this.