Easy way to make a metadata schema #1956
Description
I find it hard to remember what the specific way to make a simple schema is. We should have function like tskit.json_schema or something that returns tskit.MetadataSchema({"codec": "json"}). Or perhaps we want some sort of "SchemaBuilder" API that lets people build their schemas incrementally, adding properties? I don't think we need to support the full complexity of JSON schema, something simple that just supported adding a few (unnested) properties with documentation and a boolean required/not required flag would fit 99% of use cases. For anything more complex we can say "see the JSON schema spec".