feat: Add projectId getter to GoogleCredentials (#1813)

* feat: Add projectId getter to GoogleCredentials

* chore: Fix lint issues

* chore: Add javadoc for public method

Author: lqiu96

oauth2_http/java/com/google/auth/oauth2/GdchCredentials.java

@@ -276,6 +276,10 @@ static String getIssuerSubjectValue(String projectId, String serviceIdentityName
     return String.format("system:serviceaccount:%s:%s", projectId, serviceIdentityName);
   }
 
+  /**
+   * @return the projectId set in the GDCH SA Key file or the user set projectId
+   */
+  @Override
   public final String getProjectId() {
     return projectId;
   }

oauth2_http/java/com/google/auth/oauth2/GoogleCredentials.java

@@ -514,6 +514,17 @@ public String getQuotaProjectId() {
     return this.quotaProjectId;
   }
 
+  /**
+   * The projectId value for a Credential type. Since not all GoogleCredentials subclass have a
+   * projectId associated, the projectId may be null. A subset of GoogleCredentials subclasses will
+   * override to return their projectId.
+   *
+   * @return the project id for a Credential type
+   */
+  public String getProjectId() {
+    return null;
+  }
+
   /**
    * Indicates whether the credentials require scopes to be specified via a call to {@link
    * GoogleCredentials#createScoped} before use.

oauth2_http/java/com/google/auth/oauth2/ServiceAccountCredentials.java

@@ -798,6 +798,10 @@ public final String getServiceAccountUser() {
     return serviceAccountUser;
   }
 
+  /**
+   * @return the projectId set in the SA Key file or the user set projectId
+   */
+  @Override
   public final String getProjectId() {
     return projectId;
   }