Trigger for unit characteristics should support specs in Arena

[JasonAsmk]

Is your feature request related to a problem? Please describe.
I was trying to make a weakaura to show the enemy healer's castbar, so I tried:

• show castbar in trigger 1
  AND
• enemy unit in arena is a healer spec

But there's no way to filter for specs in Player Info -> Unit characteristics -> Arena

Describe the solution you'd like
While getting the spec is awkward due to it needing to inspect, it should be possible for arena instances because we have GetArenaOpponentSpec
as well as we could add a filter for role (tank/healer/dps) provided we have the spec above with
GetSpecializationRoleByID

Additional context
Here's the ingame picture of the relevant UI

I would consider making a PR for this if you accept contributions and passes triage. Looks good for a first issue.

[InfusOnWoW]

A PR would be welcome, the trigger's definition is in Prototypes.lua.

[Knbi]

I would consider making a PR for this if you accept contributions and passes triage. Looks good for a first issue.

I haven't seen your FR before I made mine. If you're considering contributing/making it yourself, could you implement a spec check for a specific arena unit? See my suggestion: #4691