Allow exporting the schema to a file

We should allow the export of the schema structure to a file from the schema visualizer.

The file structure should use JSON and model its shape from the Neptune Analytics schema response:

* [Neptune Analytics property graph schema](https://docs.aws.amazon.com/neptune-analytics/latest/userguide/custom-algorithms-property-graph-schema.html)

Start at the schema visualizer entry point and read the linked Neptune Analytics property graph schema documentation to compare the expected response shape with the visualizer's current schema representation. Done means the schema structure can be exported as a JSON file matching that shape.

Written by the indexing model from the issue text.