[enhancement]: Node Graph: Functional metadata

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Contact Details

_No response_

### What should this feature add?

Getting my feet wet with the node graph, very cool!

My intuition tells me that the invokeai_graph metadata that is being saved to the images should contains the necessary to be copy/paste to a json file and opened via the "Load Graph" button. Even better, the node graph UI should display copy/paste buttons right off the bat. Further more, loading graphs from Images would also be a great addition.

This feature would empower the community to shared ideas and concepts very easily. ei: from posted images in discord channels. WARNING: Easy access to creative processes tends to spark engagement and makes thing more interesting.

Optional: UI settings to toggle save metadata. Default=true.
Nice to have: In case a loaded graph is using custom nodes that are not installed, show status and/or redirect to a node manager.
Call me lazy: "double click" an image should open the graph automatically, if any. I'd settle for "right click + menu" but browsing is quite time consuming in comparison.

### Alternatives

_No response_

### Aditional Content

Diffing "saved" graph vs the "metadata" shows that the "metadata" is missing stuff and it doesn't seems to be formatted properly.