I'm not aware of any addon with that flexibility to configure "per boss" and "per class" (as some executioner skills are class relevant, like NB's "Assassin's Blade" + morphs, and some are not class dependent like 2hd "Executioner") but here is a start which might help you to show when to use executioner/finish skills:
https://www.esoui.com/downloads/info...ndingTide.html
https://www.esoui.com/downloads/info...ow-Update.html
And I really doubt that an addon should show you what skills to use, especially per boss?
You should define and learn a rotation of skills on your bars, and use that always.
And then use the executioner skill once boss drops below a certian HP -> And that's not boss dependent but only HP dependent imo.
But I'm no trial player so might be wrong here.
For a better help inside dungeons and trials try out these addons:
https://www.esoui.com/downloads/info...erUpdated.html
https://www.esoui.com/downloads/info...batAlerts.html
For dungeon ingame help texts/tooltips:
https://www.esoui.com/downloads/info...eonHelper.html
For visual markers to help with positioning etc.
https://www.esoui.com/downloads/info3684-Marker.html
Or search for special addons that got the "Dungeon's name" in their name as there also exists a few explicit helper addons for certain dungeons.
Example:
https://www.esoui.com/downloads/info...eefHelper.html
https://www.esoui.com/downloads/info...oveHelper.html
Auto change of equipped item sets based on dungeon, dungeon stage and/or boss (trash groups or boss e.g.):
https://www.esoui.com/downloads/info...sWardrobe.html
Or you can use some totla UI change like Bandits UI and add some plugins like this:
https://www.esoui.com/downloads/info...Execution.html