Add required keyword and nullable annotations to http-server-csharp model properties

[aleksator]

Hi folks!

Trying out TypeSpec with http-server-csharp generation in one of the projects - enjoying the generated code so far. Great job!

The most important improvement for me is to have required and nullable annotations for C# model properties as described in #7710, which this PR implements.

[aleksator]

@microsoft-github-policy-service agree

[aleksator]

I found that after adding required to exception properties for the definition like

@error
model ConflictError {
  @statusCode statusCode: 409;
  message: string;
}

creating an instance of the exception

throw new ConflictError("message");

produces error

Error CS9035 : Required member 'ConflictError.MessageProp' must be set in the object initializer or attribute constructor.

Fixed this by adding [SetsRequiredMembers] annotation to exception constructors.