feat!: introduce JobDetail and JobKey for job scheduling

[reugn]

• Introduce JobDetail and JobKey structures for job scheduling and identification.
• Allow a particular job to be scheduled multiple times with a distinct key and custom configuration.
• Change the Execute method in the Job interface to return an error.
• Remove the Key method from the Job interface and replace it with a JobKey identifier.
• Allow jobs to be scheduled when the Scheduler is not started.
• Allow scheduled jobs to be immediately retrieved.