Generate schemars derives in Rust generator

[leighmcculloch]

What

Generate schemars::JsonSchema derives in Rust generator whenever the existing serde feature is enabled.

Why

To support generating JSON schemas of the JSON that gets generated for the types.

Close stellar/rs-stellar-xdr#348

Todo

• Test out the initial prototype
• Check if the schema is valid (ask @willemneal for help in validating it with JS tooling)
• Consider the best way to structure the features.
• Generate custom hex/hash for any fixed or var array custom types
• Tidy and chefs kiss
• Adopt @willemneal's no additional properties from feat: add wasm-pack to allow publishing to npm rs-stellar-xdr#351 (comment) (Edit: Reduces size, but results in same affect, so leaving out and we can add as an improvement.)
• Include comments from .x files (Edit: We can do this as a follow up.)
• Add json schema meta titles for union arms. (like how @willemneal did in a visitor, but add them in the original schema for the type in the generated code) (Edit: Deferred.)

Related

• Add schemars::JsonSchema derives to all types rs-stellar-xdr#347