Sync spec to logger repos

[felixbarny]

Similar to elastic/apm#231, let's also sync the spec to the individual ECS loggers.

As an MVP, we can start with the Java logger and then add others gradually.

loggers:
  - REPO: "ecs-logging-java"
    SPEC_PATH: "ecs-logging-core/src/test/resources/spec"

[felixbarny]

For reference, this is the PR that adds the schema validation to ecs-logging-java: elastic/ecs-logging-java#111