API Endpoint Provider ProxyConfig List

[Martouta]

Last part of THREESCALE-4528
Depends on the other PRs of the issue.

APIcast needs to fetch all proxy configs of a provider and doing so by fetching first all its services (just for its IDs) and then doing an API request to proxy configs for each service is just a too slow approach.

[Martouta]

To help me test:

@tenants = Account.tenants.last(2).to_a

def make_pcs(service, number)
  proxy = service.proxy
  return if proxy.proxy_configs.count >= number
  number.times do
    proxy.proxy_configs.create!(environment: ProxyConfig::ENVIRONMENTS.sample, content: ({ proxy: { hosts: ['example.com']}}.to_json))
  end
end

@tenants.each do |t|
  t.services.each do |s|
    begin
      ServiceCreator.new(service: s).call(private_endpoint: BackendApi.default_api_backend)
    rescue ThreeScale::Core::APIClient::ConnectionError, ThreeScale::Core::APIClient::JSONError
    end
    make_pcs(s, 30)
  end
end