Document what makes a good SDK server for Composition Functions.

@negz maybe this isn't the best place for this, maybe its better as a conversation in Slack.

### What's Missing?
I have a simple GPRC server based on the Function protobuf working in Java, but I would like to turn it into something reusable and possibly distributable as an SDK.

I have a few items on my checklist that I was wondering about that don't seem to be documented anywhere.

1. The Go SDK has a --insecure flag which I assume enables/disables TLS, what are the TLS requirements for crossplane to communicate with the server in a running environment? I have only tried this thing via the CLI and wondering what kind of issues I will run into when I package it up in a container and publish it.

2. The GO SDK has some utility methods to do things like add a result.
`response.Normalf(rsp, "I was run with input %q!", in.Example)`
Is there a list of what convenience function should be present in order to be considered a proper SDK?

3. I am completely removing the protobuf and remapping the objects POJOs to avoid developers having to worry about dealing with Structs in Java. Additionally I usually try to prevent generated DTOs from leaking into the business logic so that if the transport changes over time the function itself does not need to change. I map them back to a protobuf before returning the result. Thoughts?