[Feature] Add bosses 6-8 to the boss frames

[Reqlay]

Describe the feature

Would like to show bosses 6 to 8 that some other unitframe addons show like elvui.
I believe they are the same as boss1-5 as boss6 boss7 and boss8.

Describe the implementation

Have the ability to choose how many boss frames you want to see, I reckon just from 1-8 should do just fine with default set to 5.

[Krealle]

Boss frames 6-8 are unitless frames, i.e. they require special implementation and are less performant - it won't be implemented until I get around to improving handling for unitless units.

[Reqlay]

I see, fair enough.