Addition of an option to throttle the signing requests.

[hikariuk]

Even with the -mdop option set to 1 I still periodically get errors from timestamp servers, so this option attempts to add another tool to the chest to prevent this.

[hikariuk]

In theory I like this idea, but I do not think AzureSign.Core should be changed. This seems more like a change with how AzureSignTool interacts with AzureSign.Core. AzureSign.Core's job is to just sign things, unaware of its surroundings.

I'll shift the changes to the loop. Makes the changes a bit simpler too, really.

[wizgob]

Is this possible to validate this PR? I also experience throttling with my timestamp provider.