GaiaGuardianEntity & "gaia arena" seperation

[megadoxs]

Issue description

This issue is more of a proposition of an improvement. Currently, all the methods that are related to the arena in which the player fights the Gaia are inside the GaiaGuardianEntity. My proposition would be to separate them or effectively remove all the methods that are related to the arena itself from GaiaGuardianEntity and move it to a new class (GaiaArena for example) that would hold all these methods. While this change not only simply makes sens, because after all the arena is not part of the entity, but used for its summoning, it would allow for addons to add their own bosses to the arena. To be fair, it is already possible for an addon to add a new boss to the "gaia boss fight arena", but for it to be done, a lot of methods from the GaiaGuardianEntity class have to be copied to reproduce the logic behind spawning the gaia if the arena is complete. So moving these methods that don't need to be inside GaiaGuarduanEntity and making them public instead of private, would simplify the process of adding a boss (like a third Gaia) to the arena that the Gaia uses.