Create GitHub pipeline for validating OpenAPI schema

[lukashornych]

Using GitHub action and docker, we could validate OpenAPI schemas from evitadb.io catalogs using redocly.

[lukashornych]

Checking result of validation: https://www.redhat.com/sysadmin/exit-codes-demystified

[lukashornych]

One way of doing this is to extract evita schemas from evita demo a store it in test resources in source code. Then create JUnit test which would use these schemas to start evita from which we could export OpenAPI schema into YAML. This YAML could then be passed into redocly for validation a exit code of redocly would be the final result.