add 'Button' class and 'setButtons' method for RichPresence

[Niwer1525]

This PR introduces a fluent, Java-based API for constructing Discord Rich Presence buttons.

Currently, adding buttons requires manual JSON manipulation or direct interaction with the GSON library. This change abstracts those implementation details, allowing developers to define buttons using standard Java objects, improving type safety and code readability. (While maintaining backward compatibility for those who prefer to use JSON-based approach).

Input Validation:

• null variables.
• Empty strings for labels or URLs.
• Max label and URLs sizes.

presence_builder.setButtons( // varargs
 new Button("Join Me", "https://example.com"),
 new Button("Another Cool Button", "https://example.com")
);

I hope this addition proves useful! If you have any questions or feedback regarding the implementation, please don't hesitate to reach out.