Allow passing of parameters to sub-workflows from root-level workflow inputs and step outputs

[dustinblack]

Please describe what you would like to see in this project

With the implementation of foreach loops of sub-workflows with #71 we find a usability limitation that should be addressed. The current implementation expects the complete sub-workflow schema input requirements to be passed in whole via each list item from the parent workflow input. This causes potentially a lot of duplication of input parameters that can be cumbersome and error-prone. Furthermore, a workflow author should be able to use the output of a plugin in the parent workflow as part of the input to the sub-workflow.

Please describe your use case

Global input example

The workflow author should be able to accept inputs for a benchmark workload once at a global (parent workflow) level that can be passed directly to the sub-workflow(s) where the benchmark plugins are called in order to avoid the need to provide unchanging inputs for each loop iteration.

I would like to provide this:

global_param_a: foo
global_param_b: bar
loops:
  - param_c: 1
    param_d: 2
  - param_c: 10
    param_d: 20

Instead of the current:

loops:
  - param_a: foo
    param_b: bar
    param_c: 1
    param_d: 2
  - param_a: foo
    param_b: bar
    param_c: 10
    param_d: 20

Plugin output example

The workflow author should be able to run the kubeconfig plugin once in the parent workflow and pass the connection output to the sub-workflow(s) for orchestrating.

input:
  root: RootObject
  objects:
    RootObject:
      id: RootObject
      properties:
        kubeconfig:
          type:
            type_id: string
          required: true
        workloads:
          type:
            type_id: list
            items:
              type_id: object
              id: workload
              properties:
                foo:
                  type:
                    type_id: string
                  required: true

steps:
  kubeconfig:
    plugin: quay.io/arcalot/arcaflow-plugin-kubeconfig:0.2.0
    input:
      kubeconfig: !expr $.input.kubeconfig
  workload_loop:
    kind: foreach
    items: !expr $.input.workloads
    # I should be able to pass '!expr $.steps.kubeconfig.outputs.connection' here somehow
    workflow: sub-workflow.yaml
    parallelism: 1