(feature, csharp): Add RequestOptions

[amckinney]

This introduces the RequestOptions type, which exposes the same set of properties as the ClientOptions, but does not include any default values. That way, any option explicitly set by the user at the request call-site will take precedence over the options set on the client constructor.

For example,

var user = client.GetUserAsync(
  new GetUserRequest {
    Username = "john.doe"
  },
  new RequestOptions {
      BaseUrl = "https://localhost:3000"
  }).Result;

This also updates the TimeoutInSeconds option (currently unused) to be a TimeSpan. This is more idiomatic and gives the user more control over exactly how they want to construct the time span before it's passed into the HTTP client.

[dcb6]

🙌