fix(clustering/rpc) support cluster_use_proxy option for clustering rpc protocol

[chobits]

Summary

The original hybrid mode connections like full sync (sync v1) support forward proxy via the option cluster_use_proxy. While clustering RPC protocol does not support this, this commit introduces this feature to RPC protocol.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix KAG-5555

[chobits]

Logic comes from kong/clustering//utils.lua: local ok, err = c:connect(uri, opts).

kong/kong/clustering/utils.lua

Lines 86 to 95 in 5e6b1f0

	if conf.cluster_use_proxy then
	local proxy_opts = parse_proxy_url(conf.proxy_server)
	opts.proxy_opts = {
	wss_proxy = proxy_opts.proxy_url,
	wss_proxy_authorization = proxy_opts.proxy_authorization,
	}
	ngx_log(ngx_DEBUG, _log_prefix,
	"using proxy ", proxy_opts.proxy_url, " to connect control plane")
	end

[chobits]

record flakiness: https://github.com/Kong/kong/actions/runs/12151750267/job/33886933473?pr=13971, then rerun

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