Add MetaDetails to MediaAttachment

[abraham]

No description provided.

[trwnh]

i'm not sure about this change because the meta information is serialized from whatever media processor is processing the media attachments. this information has changed significantly before. the changes are not documented, but the structure is not strictly part of the API contract, and we don't have a custom serializer for it, nor do we have a MetaDetails entity:

https://github.com/mastodon/mastodon/blob/80c8a847409c73c4e9a086c8f398a984cf5c2137/app/serializers/rest/media_attachment_serializer.rb#L48-L50

the meta information defined in the model is like so, and anything other than that is not guaranteed:

https://github.com/mastodon/mastodon/blob/80c8a847409c73c4e9a086c8f398a984cf5c2137/app/models/media_attachment.rb#L53-L58

[abraham]

These attributes are all nullable so there is no change to the existing implicit API contract. This just makes it easier for client developers to try and use the attributes when they are available.