Support adding operations to @extendable resources?

[robin-aws]

Split off from #544.

@extendable marks a resource as client-implementable. At a minimum that changes the rules about backwards compatibility: with a non-extendable resource a service is free to add operations later, but if client implementations exist for an extendable resource, they will break. At a minimum we should implement this validation rule for @extendable as part of #315.

This issue is about whether we can in fact support adding operations by including a default implementation for them. I have a question mark on the issue title because I'm pessimistic: SOME target languages support this, for example Java allows default implementations of interfaces. But we haven't designed the codegen for this case and I suspect at least one language has us locked in already. It's also likely that supporting this would lead us to less idiomatic and user-friendly choices for extendable resources, so it might be better to just accept this tradeoff in the design of @extendable resources.