Classic/TBC/Wrath: add groupRole and fetchRole to BuffTrigger2

[NoM0Re]

Description

This adds groupRole and fetchRole support to BuffTrigger for Classic/TBC/Wrath.

The functionality is already used in other triggers and code paths; this change mainly aligns BuffTrigger2 with existing behavior for consistency.
Role detection is based on the standard WoW API:
UnitGroupRolesAssigned

I tested this mainly for availability to ensure it works correctly in Classic/TBC/Wrath.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[InfusOnWoW]

If I recall correctly from the last time I looked at this, the api exist, but the user interface doesn't. Which seemed to indicate that the feature would be more confussing then useful for classic

[NoM0Re]

I've tested this now on Classic, and as well on TBC and Wrath and can confirm that the user interface exists and is fully functional alongside the API.
On Classic, the interface was added around August last year.