extensions: Update AddTransientHttpErrorPolicy once Polly v7 is released

I’m not sure if you are already aware of this, but Polly v7 will distinguish between RetryPolicy and AsyncRetryPolicy. Since all methods on the HttpClient are async, it would make sense if AddTransientHttpErrorPolicy only accepted AsyncPolicy (note: not IAsyncPolicy since the sync version of RetryPolicy also implements that interface for some weird reason).

https://github.com/App-vNext/Polly/wiki/Polly-v7-breaking-changes