MessageParameter content is always a String

[ADiamondDeveloper]

Currently, MessageParameter content is always a string. Could you update it to allow more flexibility, similar to the content in createMessage, since messages might need to handle different types of content?

From: let content: String

To: let content: [ContentPart], allowing for text, image_file, and image_url objects."

[jamesrochabrun]

Fixed #92