Idea for RFC to use an expression to include/exclude parts of a template

[mwiebe]

This idea builds on #79, it needs the ability to provide boolean expressions using the template expression substitution {{...}} syntax.

When writing a job template, you're defining an interface with parameters to control what should happen when the job runs. There are some parts of a job that would be useful to control this way, that currently you can't:

• Control whether a particular step runs or not. E.g. you may want an optional step to encode a video or not, and a parameter to control it. See Feature Request: ability to skip a step in Deadline Cloud via boolean param #80.
• Control whether a particular job or step environment runs or not. Same as for controlling the step, but for environment onEnter/onExit.
• Control whether specific host requirements are active or not. You could implement a job template that supports either CPU-only or GPU-accelerated modes, and select which host requirements to use based on a parameter.

For each case, we could add a new field "enableIf" that contains an expression evaluating to either true or false. An example step could look like:

...
parameterDefinitions:
- name: ShouldEncodeVideo
  type: BOOL
...
- name: CreateVideoFromRender
  # This is a new field. If it evaluates to true, the step runs as part of the job. If it evaluates to false, it does not.
  enableIf: "{{Param.ShouldEncodeVideo}}"
  dependencies:
  - dependsOn: RenderScene
  script:
    actions:
      onRun:
        command: bash
        args: [ "{{Task.File.Encode}}"]
...