Add schemars integration

[DXist]

This PR adds integration with schemars

The integration derives JsonSchema trait for ValidationErrors, ValidationErrorKind and ValidationError types.

This allows to include ValidationErrors in OpenAPI operation definitions as result types. Example integration code is included as an integration test that uses rocket and rocket_okapi to define an API method that takes validated input result and returns OperationError::ValidationErrors error variant in case of validation failure.

Without this integration I used newtype wrappers for the mentioned types + From definitions that did several field clones. With integration no extra memory allocations is required.

Below is a visualisation of an API error type that has recursive ValidationErrors variant. The visualisation is done by rapidoc.

[DXist]

Unfortunately Rust 1.57 is required to run test for this feature.

[IniterWorker]

Hi,

I fast reviewed @DXist's commits in #226. Impl schemars is good to me. But what are your thoughts @Keats and @krojew, about tests/examples with a hard coupling on rocket and rocket_okapi.
Perhaps too hard coupled on third-party libs rocket and rocket_okapi?

[DXist]

Hi,

Thank you for feedback!

I think it's possible to introduce a separate crate in this repository to run integration tests. And isolate test dependencies there. They can run separately with newer Rust version.