Handlers are registered on the manager using `(*Manager).RegisterSingleHandler(id HandlerID, h SingleHandlerFn, subroute ...string)`.

The server handler function has this signature: `type SingleHandlerFn func(payload []byte) ([]byte, *RemoteErr)`.

Sample handler:
```go
    handler :=  func(payload []byte) ([]byte, *grid.RemoteErr) {
        // Do something with payload

    .heldCertificate(serverCertificate, intermediateCertificate.certificate())
    .build();
```

The client only needs to know the trusted root certificate. It checks the server's certificate by
validating the signatures within the chain.

```java
HandshakeCertificates clientCertificates = new HandshakeCertificates.Builder()
    .addTrustedCertificate(rootCertificate.certificate())
    .build();

  "requestHeader": {
    "Authorization": "AWS4-HMAC-SHA256 Credential=minio/20211008/us-east-1/s3/aws4_request,SignedHeaders=host;x-amz-content-sha256;x-amz-date;x-amz-decoded-content-length,Signature=4c60a59e5eb3b0a68693c7fee9dbb5a8a509e0717668669194d37bf182fde031",
    "Content-Length": "380",
    "Content-Type": "application/octet-stream",

> Host: localhost:9000
> User-Agent: curl/7.81.0
> Accept: */*
>