[Validation] Add validation for screenshot 'size'

The `size` field for screenshots in package manifests is often incorrect. This field is a string representation of the image dimensions (e.g., "1200x900"), but it frequently does not match the actual `width` and `height` of the image file.

I don't actually know if anything consumes the `size` so I'm not sure of the impact of incorrect values.

### Statistics

Here is a breakdown of the current state of the data across existing packages in elastic/integrations@1ddb689356578e7932f98d7b46ded551e64cbdba.

Package-level screenshots:

* Out of 842 screenshots reviewed, 697 (82.8%) have an incorrect `size` value.
* Only 144 (17.1%) have a correct `size` value.

Policy template-level screenshots:

* Out of 81 screenshots reviewed, 56 (69.1%) have an incorrect `size` value.
* Only 25 (30.9%) have a correct `size` value.

### Validation

The package specification should include a validation rule that ensures the `size` field in the manifest (`width`x`height`) matches the actual dimensions of the corresponding screenshot file.

This will prevent new packages or updates from introducing incorrect data and will encourage developers to fix existing issues.