Push changes to ModuleContext down to the runner

[alecthomas]

Currently we pull the ModuleContext once, but we really want some aspects of it to be dynamic, in particular secrets/config.

We'd need to change the Controller endpoint to be a streaming response, and use PG's pubsub support to notify the controllers of changes.

1. Turn GetModuleContext() into a streaming RPC - this will also need to retry the streaming RPC in case the stream dies - potentially just use RetryStreamingClientStream if it's suitable.
2. Block until the first ModuleContext arrives so the module has something to use.
3. Inside controller.Service.GetModuleContext, periodically reconstruct the ModuleContext, hash it, and check against the previous version. If it's changed, send it. ModuleContext needs to be deterministically hashable.
4. Add a new type to the modulecontext package that takes the gRPC client and refreshes an atomically (alecthomas/atomic.Value[T]) retrievable ModuleContext.
5. Instead of adding the ModuleContext directly to the context, we'll add this self-refreshing type.
6. Inside the go-runtime replace all direct uses of ModuleContext with this new type.