[10.x] Add timestamps casting and mutation support to Eloquent

[amir9480]

This PR fixes #47769.

This PR adds a new method (freshTimestampForAttribute) to the Eloquent which can be used for creating fresh timestamp for any attribute considering either its mutation or casting and It's a replacement for the freshTimestampString method. so it will not ignore the casting or mutation of the model's attribute for any purpose.
This method can be used for other parts like SoftDeletes and touching related models instead of freshTimestampString.
For now I just used it to fix issue #47769.

Got the idea from @timacdonald PR (#47942).

[taylorotwell]

Going to look at @timacdonald's PR. If this is different explain the alternatives.

[amir9480]

@taylorotwell
This PR supports other methods like upsert and touch for any attribute. created_at attribute is also supported and can add support for deleted_at in the future without limitations.

As @driesvints mentioned in the issue, freshTimestampString was the problem so I replaced it via another method.