Use named arguments for options classes and make all fields private

[tiholic]

Python allows kwargs to pass all the possible options to create a ClientOptions object. With flutter, we need to have properties of all such objects (ClientOptions, ChannelOptions, etc) as public and SDK users need to instantiate the class and then set these properties.

So, as per the current implementation:

//usage 1
final options = ClientOptions();
options.environment = 'sandbox';
options.logLevel = LogLevel.verbose;

//usage 2
final options = ClientOptions()
    ..environment = 'sandbox'
    ..logLevel = LogLevel.verbose;

It would be more intuitive to dart developers if we can consider changing the constructors to use named arguments and make all the fields as private.

With the named arguments approach, the usage would be:

final options = ClientOptions(
  environment: 'sandbox',
  logLevel: LogLevel.verbose,
);