Create abstraction around `JobExecutionContext` to hide some implemenation details

[linkdotnet]

In #53 by @falvarez1 the JobExecutionContext has some internal details we don't want to expose. That includes the whole construction of the object itself.

To keep the current contract stable and let users be able to test their code, we had to add a ctor that mimics the old behavior. That isn't a very nice way. The much cleaner approach is to create a IJobExecutionContext interface with the public stuff and make JobExecutionContext internal.

ToDo

• [ ] Create IJobExecutionContext interface with the necessary stuff and make JobExecutionContext internal
• [ ] Update the docs
• [ ] Create an entry for a migration