Essentially this proposal OAI/Overlay-Specification#238

We should add:

• A new enum named ParameterValueSource with the members Inline and Environment (marked as experimental with code BOO002)
• A new type named OverlayParameter with the following properties: Source (previous enum), Values (List?) Separator (string?), Name (string) (marked as experimental with code BOO002)
• A new Parameters (List?) property to the OverlayAction model (marked as experimental with code BOO002)
• Serialization and deserialization support for all those things, with unit tests (the parameters collection should be using x-parameters for the time being)
• String interpolation support for any parameter value (syntax is ${parameterName}) in the overlay action object.
• Sourcing the environment variables values for the parameters of that source.
• Projecting multi-valued parameters actions as a matrix.
• Documentation of the experimental feature in the readme in the appropriate section.