Feature: Replace auth token with supplier.

Use case: oauth2 access limited time tokens which should be updated on regular basis. Supplier allows us to move refresh logic outside client.
Use specified setter to set up supplier after gitlab api object creation.

src/main/java/org/gitlab4j/api/GitLabApi.java

@@ -5,6 +5,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.WeakHashMap;
+import java.util.function.Supplier;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -711,6 +712,14 @@ public class GitLabApi implements AutoCloseable {
         return (apiClient.getAuthToken());
     }
 
+    /**
+     * Set auth token supplier for gitlab api client.
+     * @param authTokenSupplier - supplier which provide actual auth token
+     */
+    public void setAuthTokenSupplier(Supplier<String> authTokenSupplier) {
+        apiClient.setAuthTokenSupplier(authTokenSupplier);
+    }
+
     /**
      * Get the secret token.
      *

src/main/java/org/gitlab4j/api/GitLabApiClient.java

@@ -10,6 +10,7 @@ import java.security.cert.CertificateException;
 import java.security.cert.X509Certificate;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Supplier;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -60,7 +61,7 @@ public class GitLabApiClient implements AutoCloseable {
     private String baseUrl;
     private String hostUrl;
     private TokenType tokenType = TokenType.PRIVATE;
-    private String authToken;
+    private Supplier<String> authToken;
     private String secretToken;
     private boolean ignoreCertificateErrors;
     private SSLContext openSslContext;
@@ -215,7 +216,7 @@ public class GitLabApiClient implements AutoCloseable {
         this.hostUrl += apiVersion.getApiNamespace();
 
         this.tokenType = tokenType;
-        this.authToken = authToken;
+        this.authToken = () -> authToken;
 
         if (secretToken != null) {
             secretToken = secretToken.trim();
@@ -293,7 +294,7 @@ public class GitLabApiClient implements AutoCloseable {
      * @return the auth token being used by this client
      */
     String getAuthToken() {
-        return (authToken);
+        return (authToken.get());
     }
 
     /**
@@ -792,7 +793,7 @@ public class GitLabApiClient implements AutoCloseable {
         }
 
         String authHeader = (tokenType == TokenType.OAUTH2_ACCESS ? AUTHORIZATION_HEADER : PRIVATE_TOKEN_HEADER);
-        String authValue = (tokenType == TokenType.OAUTH2_ACCESS ? "Bearer " + authToken : authToken);
+        String authValue = (tokenType == TokenType.OAUTH2_ACCESS ? "Bearer " + authToken.get() : authToken.get());
         Invocation.Builder builder = target.request();
         if (accept == null || accept.trim().length() == 0) {
             builder = builder.header(authHeader, authValue);
@@ -923,4 +924,12 @@ public class GitLabApiClient implements AutoCloseable {
 
         return (true);
     }
+
+    /**
+     * Set auth token supplier for gitlab api client.
+     * @param authTokenSupplier - supplier which provide actual auth token
+     */
+    public void setAuthTokenSupplier(Supplier<String> authTokenSupplier) {
+        this.authToken = authTokenSupplier;
+    }
 }