Provide a way to receive ORCA load reports from hosts

[efimki]

Title: Provide a way to receive ORCA load reports from hosts

Description:

The ClientSideWeightedRoundRobin load balancing policy needs to receive OrcaLoadReports when they are received from the upstream host.

I propose to add an API to the HostDescription to allow LB policies and other modules to configure a callback on the Host, which will be invoked when RoutingFilter gets an ORCA report.

class HostDescription {
 …

 // Invoked each time when new custom_metrics are reported for a given host.
 using OrcaLoadReportCb = std::function<absl::Status(
     HostDescription& host, const OrcaLoadReport& orca_load_report)>;

  /*
  * Install a callback to be invoked when ORCA Load report is received for the host.
  * @param callback supplies the callback to invoke.
  * @return Common::CallbackHandlePtr a handle which can be used to unregister the callback.
  */
 Common::CallbackHandlePtr addOrcaLoadReportCb(OrcaLoadReportCb callback) const PURE;

 // Return true if Host has any CustomMetrics callbacks. Used to avoid parsing ORCA header
 // if there are no callbacks that need it.
 bool hasOrcaLoadReportCb() const PURE;

 // Invokes orca_load_report_cb attached to this host with given custom_metrics.
 void invokeOrcaLoadReportCb(const OrcaLoadReport& orca_load_report);
 …
}

[tyxia]

This issue seems to be somewhat related to #34777?

Added the area tag.