add deterministic simulated Kubernetes (dsk) package

[ecordell]

Introduces dsk: an in-process simulation layer for testing Kubernetes controllers.

Check out the README for an in depth overview.

The core abstraction is the tick: a deterministic delta cycle that buffers watch events and delivers them only when the test calls Flush() or Tick(), then waits for all resulting reconciles to complete before returning. This is borrowed from VHDL delta-cycle simulation (which I have read about but never used, I could be stretching the analogy without realizing).

This makes some difficult to test controller bug classes testable:

• Convergence: tick until IsSteady() or fail at a max-tick limit, catching both slow convergence (assert on tick count) and infinite requeue loops (never reaches steady state)
• Ordering races between controllers: Buffer() returns pending events as a slice; tests can explicitly deliver them in any order or use a property-testing library to enumerate all permutations

dsk works with both fake clients (reactor path) and real clients via envtest/kind (HTTP transport path).

It also includes:

• fault injection via a chainable FaultBuilder DSL
• an asynchandler static analyzer that catches AddRateLimited calls inside reconcile handlers before Done() is called (this is almost always a bug, but it's important for the correctness of dsk that queue add calls be syncronous)