codegen: generate go clients for custom resource routes

[amalavet]

What Changed? Why?

Previously, the SDK only generated Go clients for individual resource kinds
(CRUD operations + kind-level custom routes). However, manifest versions can
also define custom routes at the routes.namespaced and routes.cluster level —
scoped to a Group/Version rather than a specific kind — and no client was
generated for those.

This PR adds codegen support for Group/Version-level custom route clients.
When a manifest version defines routes in routes.namespaced or routes.cluster,
a CustomRouteClient is now generated in the version package (e.g.
pkg/generated/v1alpha1/client_gen.go) with typed methods for each route.

Key Changes

• codegen/jennies/goclients.go: Split Generate into generateResourceClients
  (existing per-kind clients) and generateCustomRouteClients (new GV-level
  clients). The new generator iterates over manifest versions and emits a client
  file if any namespaced or clustered routes are present.
• codegen/templates/client.tmpl: New template for the CustomRouteClient
  struct, with typed *Request/*Response wrapper methods for namespaced and
  clustered routes. Uses NewCustomRouteClientFromGenerator to construct the
  client from a resource.ClientGenerator.
• resource/client.go: Added CustomRouteClient interface (NamespacedRequest /
  ClusteredRequest) and extended ClientGenerator with
  GetCustomRouteClient(schema.GroupVersion, string).
• k8s/custom_route_client.go: Concrete CustomRouteClient implementation backed
  by the existing groupVersionClient.
• k8s/client_registry.go: Wired GetCustomRouteClient into the registry so
  callers can obtain a CustomRouteClient for any registered GV.

Addresses Issue #1277

How was it tested?

• Codegen golden file tests
• Local generation against example apiserver kinds
• Manual client setup and live requests to the example apiserver