Generate empty javadoc JAR for the shaded core module

Motivation:

It looks like Maven Central has become less permissive, we used to
deploy the shaded core without a javadocs JAR but that's not accepted
anymore.

Modifications:

Generate a javadocs JAR during the build.

For now it's a dummy, empty JAR, users can refer to the javadocs of the
unshaded one. One downside is that this won't work in IDEs; if there's a
strong demand, we can look into generating the actual javadocs later.

Result:

The driver can be deployed into Maven central again.

core-shaded/pom.xml

diff --git a/core-shaded/pom.xml b/core-shaded/pom.xml
index f920f48..fd277ef 100644
--- a/core-shaded/pom.xml
+++ b/core-shaded/pom.xml
@@ -143,6 +143,22 @@
         </executions>
       </plugin>
       <plugin>
+        <artifactId>maven-jar-plugin</artifactId>
+        <version>3.0.2</version>
+        <executions>
+          <execution>
+            <id>empty-javadoc-jar</id>
+            <goals>
+              <goal>jar</goal>
+            </goals>
+            <configuration>
+              <classifier>javadoc</classifier>
+              <classesDirectory>${basedir}/src/main/javadoc</classesDirectory>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+      <plugin>
         <artifactId>maven-dependency-plugin</artifactId>
         <executions>
           <execution>

core-shaded/src/main/javadoc/README.txt

diff --git a/core-shaded/src/main/javadoc/README.txt b/core-shaded/src/main/javadoc/README.txt
new file mode 100644
index 0000000..8ec8f95
--- /dev/null
+++ b/core-shaded/src/main/javadoc/README.txt
@@ -0,0 +1,3 @@
+This empty JAR is generated for compliance with Maven Central rules.
+
+Please refer to the javadocs of the unshaded artifact (com.datastax.oss:java-driver-core).
\ No newline at end of file