json schema for serialization format

[pca006132]

It would be nice if there is a json schema, so other projects can know which fields are needed. E.g. looking at the code, I'm not sure if op is necessary.

[mwillsey]

This test shows a small example. https://github.com/egraphs-good/egraph-serialize/blob/main/tests/tiny.json

op is required.