Extract `UnobservedExceptionHandler` for scheduled background jobs into a `Arcus.BackgroundJobs.Core` project

[stijnmoreels]

Is your feature request related to a problem? Please describe.
Currently, multiple background jobs uses the same exception handler implementation when registering the job.

Describe the solution you'd like
It would be good to extract this implementation into a Core project and reuse/maintain it in a single place.

Describe alternatives you've considered
Leave it as-is.

Additional context
First mentioned in #87 (comment)

[stijnmoreels]

Hopefully, we can benefit from an update to make this unobservable exception handling better: kdcllc/CronScheduler.AspNetCore#47