Add Amazon.Lambda.DurableExecution.Testing design doc

Spec for a testing SDK companion to Amazon.Lambda.DurableExecution:
local DurableTestRunner with in-memory orchestration, CloudDurableTestRunner
for integration tests, step inspection API, sibling-function registration,
and safety limits. Adopts the IDurableServiceClient seam (Approach B) that
Python and JavaScript reference SDKs converged on.

Author: GarrettBeatty