Add render API

[Archomeda]

• Adds the render API, along with /v2/outfits and /v2/pets.
• Fixes .NET Core 3.0 Preview 7 builds in relation to NullableContextOptions
• Consolidates a bunch of methods that accepted a CancellationToken as overload into an optional parameter
• Adds a new root client (Gw2Client) in favor of the old one (Gw2WebApiClient)
• Removes public constructors of all clients
• Changes all URL properties into a special class that allows access to the download immediately
• The BaseEndpointClient no longer contains the endpoint implementation methods on its own; it's been moved into another class and is being used as composition instead
• And has a bunch more refactoring probably