Add game event handler for `JOIN_GAME` event

[kuv2707]

Description

Following the pattern of THROW_CARD, we need to have a handler for the JOIN_GAME event. This should reside in a file in uno-game-engine/events.

Specs

The event handler should validate the event, apply it on the respective game, and return a result indicating whether the event was successfully applied or some error occured.

Notes

• Be mindful of subtle corner cases and bugs.
• Take the existing handlers as a reference to know how handlers are supposed to work.
• Refer to the type of the event object for the event.

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[ritwik-69]

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