Metadata schema

For useful data interchange with tskit-python, we need to define metadata schema.

There are currently several points slowing down a rust API for schema:

• It is not clear which serde formats are compatible with metadata on the Python side.
• Experiments have shown that serde_json works with tskit-python.
  • Ideally, we would also like a binary format compatible with the Python struct module.
• However, we have not found a solution eliminating the need to manually write the schema as a string and add it to the tables. Various crates to generate JSON schema from rust structs return schema that are over-specified and fail to validate in tskit-python.
• We also have the problem that we will need to add some Python to our CI to prove to ourselves that some reasonable tests can pass.