chore: adds 'watch' verb into inference pool reader cluster role (#1477)

**Description**

In #1377, we dropped a few verbs from the RBAC for EG to read inference
pools. Although the tests have been passing, EG was emitting errors
saying the 'watch' verb was missing in its role.

> Failed to watch {"runner": "provider", "reflector":
"pkg/mod/k8s.io/[email protected]/tools/cache/reflector.go:290", "type":
"inference.networking.k8s.io/v1, Kind=InferencePool", "error":
"inferencepools.inference.networking.k8s.io is forbidden: User
"system:serviceaccount:envoy-gateway-system:envoy-gateway" cannot watch
resource "inferencepools" in API group "inference.networking.k8s.io" at
the cluster scope"}

So this commit brings it back to the generated ClusterRole.

**Related Issues/PRs (if applicable)**

Closes #1472

Signed-off-by: Takeshi Yoneda <[email protected]>

Author: mathetake

manifests/charts/ai-gateway-helm/templates/envoy_gateway_cluster_role_for_inference_pool.yaml

@@ -19,6 +19,7 @@ rules:
     verbs:
       - "get"
       - "list"
+      - "watch"
 ---
 apiVersion: rbac.authorization.k8s.io/v1
 kind: ClusterRoleBinding