Updated CredentialsSupplier interface (made it functional interface)

Author: artem-v

services-api/src/main/java/io/scalecube/services/auth/CredentialsSupplier.java

@@ -2,16 +2,13 @@
 
 import io.scalecube.services.ServiceReference;
 import java.util.Map;
+import java.util.function.Function;
 import reactor.core.publisher.Mono;
 
+/**
+ * Returns credentials for the given {@link ServiceReference}. Credentials are being returned in
+ * most generic form which is {@code Map<String, String>}.
+ */
 @FunctionalInterface
-public interface CredentialsSupplier {
-
-  /**
-   * Returns credentials for the given {@link ServiceReference}.
-   *
-   * @param serviceReference target serviceReference
-   * @return mono result
-   */
-  Mono<Map<String, String>> getCredentials(ServiceReference serviceReference);
-}
+public interface CredentialsSupplier
+    extends Function<ServiceReference, Mono<Map<String, String>>> {}

services-transport-parent/services-transport-rsocket/src/main/java/io/scalecube/services/transport/rsocket/RSocketClientTransport.java

@@ -88,7 +88,7 @@ private Mono<Map<String, String>> getCredentials(ServiceReference serviceReferen
             return Mono.just(Collections.emptyMap());
           }
           return credentialsSupplier
-              .getCredentials(serviceReference)
+              .apply(serviceReference)
               .switchIfEmpty(Mono.just(Collections.emptyMap()))
               .doOnSuccess(
                   creds ->