Add ControlPlaneRequestEncoder

[fabianfett]

Add a new ControlPlaneRequestEncoder that encodes all requests into an existing buffer.

Motivation:

We want to write our request to the ControlPlaneAPI into one buffer, that we can reuse over and over.

Modifications:

• Add ControlPlaneRequestEncoder that writes ControlPlaneRequests into a buffer that is reused.

Result:

• Fewer messages on the ChannelPipeline.