Adding a new static ::fromTime($time) method to AbstractDate.

This will enable users to quickly set "future" headers (e.g., Expires):

```php
$headers->addHeader(Expires::fromTime('+12 hours'));
```

Includes tests :)
Adding type int|string to docblock
Splitting ->fromTime() into ->fromTimeString() and ->fromTimestamp(), and updating tests accordingly.
#6338 - minor CS fixes in the `AbstractDate` docblocks/code, simplified logic as well
Merge branch 'feature/#6338-abstract-date-header-from-time-and-datetime' into develop

Close #6338