Update existing image matching to also check against alt/captions.

The function `maybe_get_existing_attachment_id()` ([code](https://github.com/Automattic/newspack-migration-tools/blob/1d8ce878adeb414be4ae4f0017cb46a97dd86945/src/Logic/Attachments.php#L137)) will check if an existing image exists.

It would be nice to also make sure the alt/caption is also the same before assuming an image is the same. Sometimes an image can have the same binary data `md5_file()` but we may need store two separate images in the database if they have different meta data (alt, caption, etc).

To make this change, we'd need to pass in `$args` info [here](https://github.com/Automattic/newspack-migration-tools/blob/1d8ce878adeb414be4ae4f0017cb46a97dd86945/src/Logic/Attachments.php#L97) so we can do a comparison.

FYI: For the cases where we know for sure we need multiple images created (ie: no comparison is possible at image creation time) we created a way to skip the `maybe_get_existing_attachment_id()` completely. That PR was [merged here](https://github.com/Automattic/newspack-migration-tools/pull/66).

Start in src/Logic/Attachments.php by reading maybe_get_existing_attachment_id() and its caller, including the $args flow described in the issue. Trace how the existing image is selected, then verify that matching considers the relevant alt and caption metadata before reusing an attachment; done means images with different metadata are kept separate while identical metadata can still reuse the existing image.

Written by the indexing model from the issue text.