Move `using`s outside the namespace defintion

[rynowak]

Describe the proposal

The dotnet-sdk codebase uses the convention of placing usings 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 usings 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 😛