Annotate functional interfaces with FunctionalInterface.

Closes gh-204.

spring-vault-core/src/main/java/org/springframework/vault/authentication/AppIdUserIdMechanism.java

@@ -23,6 +23,7 @@ package org.springframework.vault.authentication;
  * @author Mark Paluch
  * @see AppIdAuthentication
  */
+@FunctionalInterface
 public interface AppIdUserIdMechanism {
 
 	/**

spring-vault-core/src/main/java/org/springframework/vault/authentication/ClientAuthentication.java

@@ -25,6 +25,7 @@ import org.springframework.vault.support.VaultToken;
  *
  * @author Mark Paluch
  */
+@FunctionalInterface
 public interface ClientAuthentication {
 
 	/**

spring-vault-core/src/main/java/org/springframework/vault/authentication/SessionManager.java

@@ -29,6 +29,7 @@ import org.springframework.vault.support.VaultToken;
  * @see SimpleSessionManager
  * @see ClientAuthentication
  */
+@FunctionalInterface
 public interface SessionManager {
 
 	/**

spring-vault-core/src/main/java/org/springframework/vault/client/VaultEndpointProvider.java

@@ -22,6 +22,7 @@ package org.springframework.vault.client;
  * @author Mark Paluch
  * @since 1.1
  */
+@FunctionalInterface
 public interface VaultEndpointProvider {
 
 	/**

spring-vault-core/src/main/java/org/springframework/vault/core/RestOperationsCallback.java

@@ -23,6 +23,7 @@ import org.springframework.web.client.RestOperations;
  *
  * @author Mark Paluch
  */
+@FunctionalInterface
 public interface RestOperationsCallback<T> {
 
 	/**