Better HttpClient Utilization

Previous versions of the postmark-dotnet library would instantiate multiple instances of the HttpClient. This pattern is not recommended by the .net team, and could cause some types of resource exhaustion in high concurrency use cases.

This version of the postmark library will now create a single HttpClient when you instantiate your first PostmarkClient or PostmarkAdminClient instance, and reuse the instance as needed for the lifetime of your application.

This version is largely compatible with previous versions of the client, however, the request Timeout option is no longer available during client instantiation (the timeout is set to 30 seconds).

Additionally, the NuGet package now only targets netstandard2, which should be largely compatible with recent versions of .net.