Introduce Cancellation Token support across all public service APIs. (Resolves #238)

[pianomanjh]

• ShopifyService ExecuteXXX calls leverage a cancellation token for the underlying HttpClient request and the execution policy timeout
• All existing Service public API methods now take an optional CancellationToken

I included some tests on the ShopifyService just to ensure the token is leveraged properly. I did not add tests to every single service API call, as they proxy requests to the base class, and token use there is tested. Therefore, please review the public service calls to ensure I didn't miss anything.

[clement911]

Nice!

[nozzlegear]

Awesome, thank you for all the work on this! I know it was a lot of tedious changes. Sorry it took so long to merge in, I'll let the tests run and get it published soon.

[clement911]

thank you @pianomanjh for your contribution!

[nozzlegear]

Just a heads up, this seems to be randomly throwing both OperationCanceledException and TaskCanceledException, it's not consistent at the moment. Not sure if that's an implementation issue, or if it's just the nature of cancelling tasks.

[pianomanjh]

Yeah, that is likely a difference between the exceptions thrown by consumers of the token. We pass it into both a Task.Delay and HttpClient. Both use internal timers on likely differing polling intervals to check the token. Could vary by whichever one 'notices' the cancellation first. I'll take a better look at it tonight and report back.

[nozzlegear]

Published in 5.1.0 on nuget. Thanks for the PR and all the work you put into this one!