messageField.visible param added

[victor-suri]

Created the MessageField param related to the visible property in JS SDK. Currently in JS SDK we have two options:

• a boolean value or
• a conversation predicate, a function that process a value and use it as the property

In flutter we really don't need the conversation predicate, since we already have the conversation itself to pass the ChatBox when building the chat.

[bugnano]

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Regarding your comment that we don't need the conversation predicate, I'm not entirely sure about it, the Flutter SDK tries to mimic the JS SDK as much as possible, and the fact that the conversation predicate isn't required by your application doesn't necessarily mean that it will never be required by any other application that uses this SDK.

The fact that the visible parameter has been omitted until now is because Dart doesn't support union types (correct me if I'm wrong here, maybe now it does), and I have yet to find a way to implement this feature cleanly without union types.