open-api: Use openapi-generator-gradle-plugin for validating specification

[dramaticlly]

Today we have open-api specification validated as part of github actions/workflow when PR was submitted (or by author running make task)

This PR tried to use openapi-generator-gradle-plugin for open-api specification validation on build, with recommendations. I added validation to both s3-signer-open-api.yaml and rest-catalog-open-api.yaml

Good spec

./gradlew :iceberg-open-api:build
Starting a Gradle Daemon (subsequent builds will be faster)

> Task :iceberg-open-api:validateRESTCatalogSpec
Validating spec /Users/stevezhang/workspace/iceberg/open-api/rest-catalog-open-api.yaml
Spec is valid.

Bad spec

> Task :iceberg-open-api:validateRESTCatalogSpec FAILED
Validating spec /Users/stevezhang/workspace/iceberg/open-api/rest-catalog-open-api.yaml

Spec is invalid.
Issues:

        attribute .servers.foo is unexpected

        attribute .servers.url is missing


FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':iceberg-open-api:validateRESTCatalogSpec'.
> Validation failed.

[dramaticlly]

@Fokko @nastra if you want to take a look?

[Fokko]

This is great, thanks for working on this @dramaticlly