Upstream proposal: https://github.com/argoproj/argo-cd/blob/master/docs/proposals/multiple-sources-for-applications.md

Add optional ref field to Application Source

For making files accessible to other sources, add a new ref field to the source. For example, in below ApplicationSpec, we have added ref: myRepo to the my-repo repository and have used reference $myRepo to the elasticSearch repository.

spec:
  sources:
    - repoURL: https://github.com/my-org/my-repo  # path is missing so no manifests are generated
      targetRevision: master
      ref: myRepo                                 # repo is available via symlink "myRepo"
    - repoURL: https://github.com/helm/charts
      targetRevision: master
      path: incubator/elasticsearch               # path "incubator/elasticsearch" is used to generate manifests
      helm:
        valueFiles:
          - $myRepo/values.yaml                   # values.yaml is located in source with reference name $myRepo 

The file should be allowed to referenced for `spec.source.helm.valueFiles` and `spec.source.helm.fileParameters`. 

Acceptance Criteria:

• "ref" variable is parsed successfully
• One source is able to fetch files from another source and Application manifest is generated successfully
• Unit tests are added and all current test scenarios pass successfully