Add support for Author Username on the Merge Request API (gitlab4j#1283)

(cherry picked from commit 12e8ebf)

Author: lucasls

gitlab4j-models/src/main/java/org/gitlab4j/api/models/MergeRequestFilter.java

@@ -49,6 +49,7 @@ public class MergeRequestFilter implements Serializable {
      */
     private Long authorId;
 
+    private String authorUsername;
     private Long assigneeId;
     private Long reviewerId;
     private String myReactionEmoji;
@@ -271,6 +272,19 @@ public MergeRequestFilter withAuthorId(Long authorId) {
         return (this);
     }
 
+    public String getAuthorUsername() {
+        return authorUsername;
+    }
+
+    public void setAuthorUsername(String authorUsername) {
+        this.authorUsername = authorUsername;
+    }
+
+    public MergeRequestFilter withAuthorUsername(String authorUsername) {
+        this.authorUsername = authorUsername;
+        return (this);
+    }
+
     public Long getAssigneeId() {
         return assigneeId;
     }
@@ -524,6 +538,7 @@ public GitLabForm getQueryParams() {
                 .withParam("updated_after", updatedAfter)
                 .withParam("updated_before", updatedBefore)
                 .withParam("scope", scope)
+                .withParam("author_username", authorUsername)
                 .withParam("assignee_id", assigneeId)
                 .withParam("reviewer_id", reviewerId)
                 .withParam("my_reaction_emoji", myReactionEmoji)