Add support for providing a custom layout during logging bootstrap

Dropwizard users have the ability to configure their application to
use JSON as the logging format. Unfortunately, the default logging
bootstrap mechanism which kicks off before Dropwizard logging
configuration, uses the default logging layout. As a result, some
internal logging messages (Jetty, Hibernate initialization) are
produced in a not recognized format.

This change add possibility to specify a layout factory during
the bootstrap process. Users can override the bootstrap method of
`Application` and provide an own layout format. For example,

```
BootstrapLogging.bootstrap(Level.INFO, (loggerContext, timeZone) ->
    new EventJsonLayoutBaseFactory().build(loggerContext, timeZone));
```

As a result, the all applications logs will produced in the JSON format.

dropwizard-json-logging/src/test/java/io/dropwizard/logging/json/LayoutIntegrationTests.java

@@ -40,7 +40,8 @@
 public class LayoutIntegrationTests {
 
     static {
-        BootstrapLogging.bootstrap(Level.INFO);
+        BootstrapLogging.bootstrap(Level.INFO, (loggerContext, timeZone) ->
+            new EventJsonLayoutBaseFactory().build(loggerContext, timeZone));
     }
 
     private final ObjectMapper objectMapper = Jackson.newObjectMapper();

dropwizard-logging/src/main/java/io/dropwizard/logging/BootstrapLogging.java

@@ -2,9 +2,11 @@
 
 import ch.qos.logback.classic.Level;
 import ch.qos.logback.classic.Logger;
+import ch.qos.logback.classic.LoggerContext;
 import ch.qos.logback.classic.filter.ThresholdFilter;
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.core.ConsoleAppender;
+import ch.qos.logback.core.Layout;
 import ch.qos.logback.core.encoder.LayoutWrappingEncoder;
 
 import javax.annotation.concurrent.GuardedBy;
@@ -35,6 +37,10 @@ public static void bootstrap() {
     }
 
     public static void bootstrap(Level level) {
+        bootstrap(level, DropwizardLayout::new);
+    }
+
+    public static void bootstrap(Level level, LayoutFactory layoutFactory) {
         LoggingUtil.hijackJDKLogging();
 
         BOOTSTRAPPING_LOCK.lock();
@@ -45,8 +51,8 @@ public static void bootstrap(Level level) {
             final Logger root = LoggingUtil.getLoggerContext().getLogger(org.slf4j.Logger.ROOT_LOGGER_NAME);
             root.detachAndStopAllAppenders();
 
-            final DropwizardLayout formatter = new DropwizardLayout(root.getLoggerContext(), TimeZone.getDefault());
-            formatter.start();
+            final Layout<ILoggingEvent> layout = layoutFactory.create(root.getLoggerContext(), TimeZone.getDefault());
+            layout.start();
 
             final ThresholdFilter filter = new ThresholdFilter();
             filter.setLevel(level.toString());
@@ -57,7 +63,7 @@ public static void bootstrap(Level level) {
             appender.setContext(root.getLoggerContext());
 
             final LayoutWrappingEncoder<ILoggingEvent> layoutEncoder = new LayoutWrappingEncoder<>();
-            layoutEncoder.setLayout(formatter);
+            layoutEncoder.setLayout(layout);
             appender.setEncoder(layoutEncoder);
             appender.start();
 
@@ -67,4 +73,10 @@ public static void bootstrap(Level level) {
             BOOTSTRAPPING_LOCK.unlock();
         }
     }
+
+    @FunctionalInterface
+    public interface LayoutFactory {
+
+        Layout<ILoggingEvent> create(LoggerContext loggerContext, TimeZone timeZone);
+    }
 }