Adding a user agent via the HttpClientBuilder

Adding a user agent configuration field to HttpClientConfiguration, and a default user agent to HttpClients, consisting of `{serviceName} ({clientName})`.

In our environment we often wonder where requests are coming from. Using the source IP is less visual, requires being able to map that IP to something meaningful, and requires load balancers to be correctly configured to send a `HTTP_X_FORWARDED_FOR` header. By setting the user agent we can include it in the request logs and easily identify traffic sources.