Hey dude sorry this has happened to you - a real ball ache.
Same thing happened to me !
I used a ceramic drill to drill out as much of the spark plug as I need to allow an easy out to grip the metal thread.
Soaked with penetrating spray overnight and a few other occasions.
Then I used an easy-out. it worked but was lucky, non of the garages wanted to touch it without taking the head off and drilling it out.
Tip 1) Pre drill it, otherwise you will struggle to get it to bite however only drill what you need - don't want swarf in the cylinder
Tip 2) Go slowly when using the easy out - they blunt real quick and you only get a couple of shots.
Worst case scenario ... you dont drill enough and you snap the easy out as you turn it too fast !
But it is possible to get out .. heres mine using a halford easy out [second to largest size from memory]