feat(api): safer metadata extraction during image upload

[psychedelicious]

Summary

• Add a utility to extract metadata/workflow/graph from an image.
• Fix typing for the metadata parameter int he upload route (was previously too wide, accepting any JsonValue when only stringified JSON dict is valid).
• Use the new util in the route.
• Add tests for the util.

Related Issues / Discussions

offline discussion

QA Instructions

We need to test that metadata is retained as expected after images are downloaded and then uploaded. There are 3 places to test:

Send to gallery metadata

• Generate an image in using send-to-gallery
• Download the image
• Upload it into Invoke
• Confirm that the metadata shows up
• Confirm that the graph tab of metadata still shows the graph
• Load workflow from image (which will build a workflow from this graph, it should use the auto-layout)
• Confirm the workflow looks correct - no errors

Send to canvas metadata

• Generate an image using send-to-canvas, accept it as a raster layer
• Click Save Canvas to Gallery
• Download the saved canvas
• Upload it back into Invoke
• Check the metadata - it should show the raster layer as json

Workflow Editor metadata

• Generate an image in Workflow Editor
• Download it
• Upload it
• Confirm you can still load the workflow from the uploaded image (it will load from the workflow directly and should look the same as it did when you generated the image)

Merge Plan

n/a

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)