[Moco] Create interface for user-defined `InputController`s

[nickbianco]

Addressing issue #3605 will introduce the notion of an InputController, which will enable controllers driven by inputs from OCP controls. This new functionality will introduce new complexities in defining, solving, and returning solutions for MocoStudys. The following considerations will need to be addressed:

• How will bounds from setControlInfo() be obeyed when the model control is no longer an OCP control?
• How will users set bounds on Input controls (e.g., setInputInfo())?
• Should we allow users to add OCP controls on top of model controls produced by controllers?
• What do we return in a MocoSolution? Both the Input values and the model control values?

Link to planning doc.