Provide way to set authentication

[domdorn]

Please include the following file in the sourcecode to allow users to set credentials:

import akka.http.scaladsl.model.headers.Authorization
import io.grpc.{CallCredentials, Metadata, Status}

import java.util.concurrent.Executor
import scala.util.{Failure, Success, Try}

object AuthorizationCallCredentials {
  import io.grpc.Metadata.ASCII_STRING_MARSHALLER
  val AuthKey: Metadata.Key[String] = Metadata.Key.of("Authorization", ASCII_STRING_MARSHALLER);

}

final case class AuthorizationCallCredentials(auth: Authorization) extends CallCredentials {
  override def applyRequestMetadata(
    requestInfo: CallCredentials.RequestInfo,
    appExecutor: Executor,
    metadataApplier: CallCredentials.MetadataApplier): Unit =
    // implemented similar to official example from:
    // https://github.com/grpc/grpc-java/blob/master/examples/example-jwt-auth/src/main/java/io/grpc/examples/jwtauth/JwtCredential.java
    appExecutor.execute { () =>
      val headers = new Metadata()
      headers.put(AuthorizationCallCredentials.AuthKey, auth.credentials.scheme() + " " + auth.credentials.token())
      Try(metadataApplier.apply(headers)) match {
        case Failure(ex) => metadataApplier.fail(Status.UNAUTHENTICATED.withCause(ex))
        case Success(_)  => ()
      }
    }

  override def thisUsesUnstableApi(): Unit = ()
}

This allows to set any credential type that is already supported by Akka-HTTP, e.g. like

  private val cred = Authorization(
    BasicHttpCredentials(
      config.getString("clients.timeseries.username"),
      config.getString("clients.timeseries.password")
    ))

  val grpcClientSettings = GrpcClientSettings.connectToServiceAt(host, port)(akka).withCallCredentials(AuthorizationCallCredentials(cred)))

Mentioning it, e.g. somewhere close to https://doc.akka.io/docs/akka-grpc/current/server/akka-http.html#example-authentication-authorization would probably also be helpful.