[keyvault] add continuation token for key polling

[iscai-msft]

fixes #12027

For context: this PR is adding a continuation token functionality, so users can pickle their pollers and restart the polling from the state where they left off. These two pollers in key vault are also quite unique, the reason being that

1. They're not defined as long running on the service side
2. When we make the initial call to the service, the final response object, i.e. a deleted key is returned. However, it takes a while for the key to be fully deleted on the service side, so we poll until the service has fully deleted the key. We do allow users to immediately get the response object from the service though (our result function doesn't call wait)

[iscai-msft]

closing, @mccoyp has his own PRs