enhancement: Metadata (headers) support

[oguzhand95]

Summary

• Added gRPC Metadata support
• Simplified the internal client-building logic

Example

When metadata ({"wibble": "wobble"}) is provided during both client initialization and CheckResources call, the cerbos audit logs show;

{
  "log.logger":"cerbos.audit",
  "log.kind":"access",
  "timestamp":"2023-12-13T17:26:56.614190Z",
  "callId":"01HHJ3F8B657J3PB3S6P21YY3X",
  "peer":{
    "address":"127.0.0.1:50993",
    "userAgent":"grpc-dotnet/2.59.0 (.NET 6.0.5; CLR 6.0.5; net6.0; osx; x64)"
  },
  "metadata":{
    "wibble":{
      "values":[
        "wobble, wobble"
      ]
    }
  },
  "method":"/cerbos.svc.v1.CerbosService/CheckResources"
}