pid_controller: add reference interfaces for gains

[christophfroehlich]

Currently, the pid_controller can be dynamically tuned by changing parameters.

Proposal: It would be useful for gain-scheduling use cases to export the gains as reference interfaces, which can then be claimed by preceding controllers.

[rehanshah17]

Could I take up this issue please?

[rehanshah17]

I think I was able to figure out how to export the pid constants as reference interfaces. Could you lead me down the right path of how the controllers would be able to claim this?

[christophfroehlich]

let us get #1601 working first.