fix: make JwtClaims.newBuilder() public (#350)

Author: chingor13

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

@@ -70,7 +70,7 @@ public abstract class JwtClaims implements Serializable {
  */
  abstract Map<String, String> getAdditionalClaims();
 
- static Builder newBuilder() {
+ public static Builder newBuilder() {
  return new AutoValue_JwtClaims.Builder().setAdditionalClaims(ImmutableMap.<String, String>of());
  }