Enable usage of Route Groups for client generation

[dev-in-disguise]

When using controllers it is possible to use {controller} when setting stuff within the OpenApiProjectReference. We use that in our projects to set <ClassName>{controller}Client</ClassName>.

Since .NET 7 when using minimal apis we can now define so called route groups via app.MapGroup("GroupName") to group several endpoints together. When using this I would now like to have the possibility to define the ClassName in a similar fashion to what {controller} does. So e.g. a {routeGroup} which creates a client per route group.

By the way I'd gladly try to implement a first version if you think this would be a good and necessary addition.