Allow for 'choice' inputs

Current situation

We are using inputs more and more (workflows, plugin descriptors, ...).

Some inputs have a predefined set of values they can take. But currently each plugin has to check for this, which means it's on the code side, and is repetitive.

Desired outcome

A way to only specify a set of values for an input:

inputs:
  foo:
    description: bar or baz
    type: choice
    options:
    - bar
    - baz

If it is not required, it must have a default (which should be in the allowed options).