Support Virtual Threads in the WorkManager

[gastaldi]

• Fixes Support Virtual Threads #147

[maxandersen]

good we are exploring the abilty of virtual threads - but I noticed the use of a global virtual thread on/off switch. Something we've avoided elsewhere (so far).

Is having a global property the right way of doing this? anywhere else we explicitly require use of annotation at the entry point to make it very clear and explicit the execution model is different.

so this is not -1'ing the idea; just checking this really is the best way to expose VT in JCA.

[gastaldi]

@maxandersen thank you for your feedback. I thought about reading that annotation from the ResourceAdapterFactory implementation, maybe that's a better idea than a global build setting.

I'll experiment a bit before marking this PR as ready