Add registry_mirror & across as a known properties

[aaron-uk]

Describe the bug

Wrongfully displaying unknown property for resource type RegistryImageSource for registry_mirror & DoStep for across

To Reproduce

check-sonarqube-quality-gate.yml

---
platform: linux

image_resource:
  type: registry-image
  source:
    repository: cathive/concourse-sonarqube-qualitygate-task
    tag: "latest"
    registry_mirror:
      host: my-registry.com

inputs:
  - name: sonarqube-analysis

run:
  path: /sonarqube-qualitygate-check
  dir: sonarqube-analysis

Sample

Unknown property 'registry_mirror' for type 'RegistryImageSource'vscode-concourse(YamlSchemaProblem)

Unknown property 'across' for type 'DoStep'vscode-concourse(YamlSchemaProblem)

registry_mirror info from concourse ci resource repo

across step info

[martinlippert]

@kdvolder Can you take a look at this? That would be great!