Take user input for Pipelines custom build

It would be useful to be able to accept user input strings/fields or choose a related git reference when kicking off a custom build.

The user input could be made available as environmental variables. The names of the fields could be defined in the yml file, eg.

#!yml

pipelines:
  custom:
    stuffwithuserinput:
      - userinput:
      	- foo
      	- bar
      - step:
          script:
            - echo $foo $bar

So far my use cases have all been doing things with two repos; so I could also use env vars that identify a repo, branch or tag based on user input (eg. dropdowns). That would have the nice side effect of checking the target exists.

I can already implicitly identify one side of that process by choosing where to trigger the build; but obviously not both. There are some ways to do build gymnastics and get around it, but it would be a heck of a lot easier to just be able to spec "this build needs that tag from that repo" when you run the custom build.