Define a mechanism to select the measurement consumer

[ocelotl]

Ok, but I think that we will need a mechanism to do that. Opening a corresponding issue.

Originally posted by @ocelotl in #2339 (comment)

[aabmass]

I don't think we need to tackle this for a while. MeasurementConsumer is not part of the spec and we don't need to publicly expose it or allow users to implement it right now. Once we have good confidence in it, maybe we should open it up, but I'd rather keep it private and have the freedom to tweak things even after a stable release.

[ocelotl]

As far as I understand, the design is based on the idea of MeasurementConsumer being a component that can be replaced by another implementation. If that is the case, then don't we need a way to let the user select the implementation? 🤷

[aabmass]

At some point that's the idea. For now, we don't need to expose any of the MeasurementConsumer stuff to the user/public API.