enhancement: Metadata (headers) support

[oguzhand95]

Summary

• Added gRPC Metadata support
• Simplified the internal client-building logic
• Moved the playground-related logic to the cerbos client builder like other SDKs

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":"2023-12-13T17:30:40.905891Z",
  "callId":"01HHJ3P3C9FBSGC94Y12NMANTR",
  "peer":{
    "address":"127.0.0.1:51027",
    "userAgent":"grpc-php/1.55.0 grpc-c/32.0.0 (osx; chttp2)"
  },
  "metadata":{
    "wibble":{
      "values":[
        "wobble",
        "wobble"
      ]
    }
  },
  "method":"/cerbos.svc.v1.CerbosService/CheckResources"
}