[Scheduler] debug:schedule refinements

[kbond]

Q	A
Branch?	6.3
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Followup to #49795
License	MIT
Doc PR	n/a

Some fixes/refinements to the output:

1. If the message is an envelope, unwrap
2. prefix stringable message/trigger with short class name

 --------------------------- -------------------------------------- --------------------------- 
  Message                     Trigger                                Next Run                   
 --------------------------- -------------------------------------- --------------------------- 
  DoSomething: do something   CronExpressionTrigger: * * * * *       2023-03-25T08:03:00-04:00  
 --------------------------- -------------------------------------- ---------------------------

[kbond]

Currently, when outputting the message, if it's \Stringable, we use that. This might not be desired...

Do you think there would be value in a DescribableMessageInterface in messenger that provides a getMessageDescription() method? There could be other usages, like using as the context in #49792.

[kbond]

Do you think there would be value in a DescribableMessageInterface in messenger that provides a getMessageDescription() method? There could be other usages, like using as the context in #49792.

Another, perhaps more palatable idea: #49792 (comment)

[fabpot]

I would keep using the toString() method for the message. For the trigger, we might decide that TriggerInterface extends Stringable.

[fabpot]

Thank you @kbond.