Move `using`s outside the namespace defintion

## Describe the proposal

The dotnet-sdk codebase uses the convention of placing `using`s inside the namespace declaration. This is an old convention that was enforced by some linting tools back in the day, and it isn't the way most modern C# code looks - especially OSS code.

The best reasons to change this:
- The other major C# OSS code bases use `using`s outside (consistency with popular projects)
- The defaults for tools like VS, Rider, `dotnet new` place `using` directives outside the namespace - our code style fights with the defaults most users will have

There **is** actually a small semantic difference between placing usings inside vs outside. If anyone can tell me what it is without looking it up, you win a free copy of Dapr 😛