fix(build): include cluster-provided load balancer extension

[lhlxc]

What this PR does / why we need it:

Adds envoy.load_balancing_policies.cluster_provided to the curated Envoy extension build configuration.

Envoy 1.28 and later convert the legacy lb_policy: CLUSTER_PROVIDED configuration into a typed load-balancer policy and look up envoy.load_balancing_policies.cluster_provided through the extension registry. The Higress Proxy allowlist did not include that factory after the Envoy 1.36 migration, so affected gateway configurations failed during startup with:

No load balancer factory found for LB type: CLUSTER_PROVIDED

Linking the existing Envoy extension restores CLUSTER_PROVIDED for Redis custom clusters and other cluster-provided load-balancer users.

Which issue this PR fixes: N/A

Special notes for your reviewer:

Validation performed:

• git diff --check
• Confirmed the referenced public target exists at //source/extensions/load_balancing_policies/cluster_provided:config

A local full //:envoy link was not completed: the repository's container build requires mounting the Docker socket, and local Bazel dependency resolution did not complete in this environment. The upstream CI build should provide the complete link validation.