Add optional schemars support for JSON Schema generation

[copumpkin]

It would be great if passkey-types supported https://crates.io/crates/schemars for JSON Schema generation, perhaps gated behind an optional feature flag.

I'm building a PassKey implementation using https://github.com/oxidecomputer/dropshot, which generates OpenAPI API definitions from code. For that to work properly with this package, the types here need to derive JsonSchema.

If you're open to it, I'd be happy to make a pull request with that support. I'd probably gate it behind a feature flag so that people who don't need it don't get the extra dependency.

[Progdrasil]

Hi @copumpkin, we won't be using this ourselves, but I'd be happy to accept PRs so long as it is properly feature gated