Make OpenTelemetryAppender.Builder public (#7521)

Allows for programatic configuration as discussed in this [CNCF slack
thread](https://cloud-native.slack.com/archives/C01N5UCHTEH/p1673028580643009).

Author: jack-berg

instrumentation/log4j/log4j-appender-2.17/library/src/main/java/io/opentelemetry/instrumentation/log4j/appender/v2_17/OpenTelemetryAppender.java

@@ -49,7 +49,7 @@ public static <B extends Builder<B>> B builder() {
     return new Builder<B>().asBuilder();
   }
 
-  static class Builder<B extends Builder<B>> extends AbstractAppender.Builder<B>
+  public static class Builder<B extends Builder<B>> extends AbstractAppender.Builder<B>
       implements org.apache.logging.log4j.core.util.Builder<OpenTelemetryAppender> {
 
     @PluginBuilderAttribute private boolean captureExperimentalAttributes;

instrumentation/log4j/log4j-appender-2.17/library/src/test/java/io/opentelemetry/instrumentation/log4j/appender/v2_17/OpenTelemetryAppenderConfigTest.java

@@ -32,6 +32,8 @@
 import org.apache.logging.log4j.Marker;
 import org.apache.logging.log4j.MarkerManager;
 import org.apache.logging.log4j.ThreadContext;
+import org.apache.logging.log4j.core.impl.Log4jLogEvent;
+import org.apache.logging.log4j.message.FormattedMessage;
 import org.apache.logging.log4j.message.StringMapMessage;
 import org.apache.logging.log4j.message.StructuredDataMessage;
 import org.junit.jupiter.api.BeforeAll;
@@ -68,6 +70,22 @@ void setup() {
     ThreadContext.clearAll();
   }
 
+  @Test
+  void initializeWithBuilder() {
+    OpenTelemetryAppender appender =
+        OpenTelemetryAppender.builder().setName("OpenTelemetryAppender").build();
+    appender.start();
+
+    appender.append(
+        Log4jLogEvent.newBuilder()
+            .setMessage(new FormattedMessage("log message 1", (Object) null))
+            .build());
+
+    List<LogRecordData> logDataList = logRecordExporter.getFinishedLogItems();
+    assertThat(logDataList)
+        .satisfiesExactly(logRecordData -> assertThat(logDataList.get(0)).hasBody("log message 1"));
+  }
+
   @Test
   void logNoSpan() {
     logger.info("log message 1");