Add Roslyn Analyser to detect common problems

[adam-resdiary]

We can potentially create Roslyn Analysers to detect common issues when people are using the runtime library or generated code. Examples of problems we can detect are:

• Returning null from a service method.
• Throwing an exception that isn't part of the service method definition.

To illustrate this, let's take the example of returning null from a service method. Thrift doesn't support null return results from service methods, and has no way to serialise that. So if someone writes the following code:

public async Task<User> GetUser(int userId)
{
    var user = this.userRepository.GetById(userId);

    if (user != null)
    {
        return this.mapper.Map<User>(user);
    }

    return null;
           ^^^^
}

We can report an error when returning null since we know that it isn't a valid return result.

An example of a project that provides analysers like this is xUnit. It can detect problems like Theories not having the correct parameters passed to them at compile time, something that would typically cause a runtime error.