fix: Making fields protected in SchedulerBuilder (#569)

## Brief, plain english overview of your changes here

Making fields protected so that we can access in custom SchedulerBuilder

## Fixes
<!--- Which issue # does this fix? --->


## Reminders
- [x] Added/ran automated tests
- [x] Update README and/or examples
- [x] Ran `mvn spotless:apply`

---
cc @kagkarlsson

db-scheduler/src/main/java/com/github/kagkarlsson/scheduler/SchedulerBuilder.java

@@ -75,11 +75,11 @@ public class SchedulerBuilder {
   protected LogLevel logLevel = DEFAULT_FAILURE_LOG_LEVEL;
   protected boolean logStackTrace = LOG_STACK_TRACE_ON_FAILURE;
   protected boolean enablePriority = false;
-  private boolean registerShutdownHook = false;
-  private int numberOfMissedHeartbeatsBeforeDead = DEFAULT_MISSED_HEARTBEATS_LIMIT;
-  private boolean alwaysPersistTimestampInUTC = false;
-  private List<SchedulerListener> schedulerListeners = new ArrayList<>();
-  private List<ExecutionInterceptor> executionInterceptors = new ArrayList<>();
+  protected boolean registerShutdownHook = false;
+  protected int numberOfMissedHeartbeatsBeforeDead = DEFAULT_MISSED_HEARTBEATS_LIMIT;
+  protected boolean alwaysPersistTimestampInUTC = false;
+  protected List<SchedulerListener> schedulerListeners = new ArrayList<>();
+  protected List<ExecutionInterceptor> executionInterceptors = new ArrayList<>();
 
   public SchedulerBuilder(DataSource dataSource, List<Task<?>> knownTasks) {
     this.dataSource = dataSource;