Move messaging_client.nim and the delivery_service machinery (send/recv
services, processors, not_delivered_storage) out of waku/ and waku/node/
into a dedicated top-level messaging/ folder.
The delivery services are a messaging-layer concern built on top of the
node, not part of node core, so a sibling folder makes that boundary
explicit. Shared message types (waku/api/types) and the node-owned
subscription_manager are left in place to avoid churn on widely-imported
modules.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
