Coders only should have a FunctionSpec, not an SdkFunctionSpec

We added environments to coders to account for "custom" coders where it is only really possible for one SDK to understand them, like this:

Coder {
  spec: SdkFunctionSpec {
    environment: "java_sdk_docker_container",
    spec: FunctionSpec {
      urn: "beam:coder:java_custom_coder",
      payload: <serialized java bytes>
    }
  }
}

But a coder must be understood by both the producer of a PCollection and its consumers. A coder is not the same as other UDF, though these are user-defined.

A pipeline where either the producer or consumer cannot handle the coder is invalid, and we will have to build our cross-language APIs to prevent construction of such a pipeline. So we can drop the environment.

I think there are some folks who want to reserve the ability to add an environment later, perhaps, to not pain ourselves into a corner. In this case, we can just add a field to Coder.