Allow specifying multi-select workflow parameters

[mvdbeek]

This just works, what's needed is:

• Only allow multiple=true connections to actual multiple=true inputs
• gxformat2 language to describe this
• test

Closes #12233 (comment) and #16091 and #17957

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[jmchilton]

Thank you for adopting the workflow tests so quick. I appreciate it! This looks great.