request: allow passing in HttpClient instance, or allowing a method to configure the HttpClient created

[jamesmanning]

Currently I'm hitting the 100 second default timeout in some cases during queries - since the ServiceNowClient ctor constructs the HttpClient itself and doesn't currently have a ctor overload to allowing passing in the HttpClient or an Action or whatever to configure it, I don't have an (easy) way of configuring the HttpClient.

I'd like to be able to configure it for things like:

• specifying proxy values in some specific test cases where I want to run the requests through a testing proxy
• change the timeout from its default of 100 seconds
• allow configuring Polly policies for things like retry/circuit breaker

Thanks!