Add ControlPlaneRequest, ControlPlaneResponse

[fabianfett]

Motivation:

In the coming weeks, we want to restructure the LambdaRuntime internals in such a way, that we allocate a lot less resources per invocation. To accomplish this, we want to encode and decode the ControlPlaneAPI requests directly from their semantic value. For this reason, we need a specialized ControlPlaneRequest and ControlPlaneResponse type that we can encode from and decode into.

Modifications:

• Add ControlPlaneRequest, ControlPlaneResponse
• Move Invocation and ErrorResponse into same file as ControlPlaneRequest
• Remove Lambda namespace around Invocation

Result:

• We can add encode methods for ControlPlaneRequest in a follow up PR
• We can add decode methods for ControlPlaneResponse in a follow up PR