feat(types): introduce ResourceNamespaceMap for monorepo namespace augmentation (#2434)

Adds `ResourceNamespaceMap` — a mergeable interface that lets each package in a monorepo declare its own namespace resource types without TS2717 collisions on `CustomTypeOptions.resources`. Per-property merge handles same-namespace contributions from multiple packages; conflict keys are silently dropped. Backwards-compatible with existing `CustomTypeOptions.resources` augmentation. Closes #2409.