Override origin on Endpoints

[jkylling]

This change sets origin on Endpoints, such that the LoadBalancedChannel can be used with proxies which route to backend services based on origin.

My use case is to be able to use the same grpc client for both DNS based load balancing, and for accessing internal gRPC services through proxies (this is useful for testing).

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Note

Set the origin on gRPC endpoints (HTTP/HTTPS) derived from service authority to support SNI/proxy-based routing.

• gRPC Endpoint Construction:
  • Set origin on tonic::transport::Endpoint in ginepro/src/service_probe.rs.
  • Add origin: http::uri::Uri to GrpcServiceProbe; initialize from ServiceDefinition.authority() and switch scheme to HTTPS in with_tls.
  • Add helper create_origin(...) to build the origin URI.
• Service Definition:
  • Add authority: http::uri::Authority to ServiceDefinition; compute in from_parts and expose authority() accessor.
• Changelog:
  • Note endpoint construction now sets origin for SNI-based servers.

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[jkylling]

This PR resolves the same issue as #51