Add more return fields on 'get_sidecar_nodes' method

[Sprizgola]

Provide us a use case of the feature
The user should call get_sidecar_nodes() or adding a list of GraphImage fields at the method call such as __get_sidecar_nodes(fields=["id", "shortcode", ....])_.
The return type should be either a GraphImage or a PostSidecarNode (as currently is)

Describe the solution you'd like
It could be a nice thing to add the fields in the instaloader.structure.Post.get_sidecar_nodes method.
The node is already in memory during the execution, so it could be a nice feature to add.

The actual implementation yield this response:

yield PostSidecarNode(is_video=is_video, display_url=display_url, video_url=node['video_url'] if is_video else None)

If the feature request is accepted, would you be willing to submit a pull request?
Yes (with some assistance during PR!)

Additional context
This feature could be a cool thing because, in my case, i need the metadata for every image without the needing to access private attribute.

[aandergr]

PostSidecarNode could be modified to contain more of the fields that are provided with the sidecar nodes. PostSidecarNode is currently implemented as a NamedTuple, however, these can be easily and backwards-compatibly refactored to a proper class, like has recently been done with PostComment.