Declare dependency on Apache commons lang3

Fixes jenkinsci#233 (at least partially)

The plugin depends on Apache commons lang3 but did not declare it
directly in the pom.

Declare it in the pom with hope that will help to resolve the plugin
bill of materials issue described at
jenkinsci/bom#2484 (comment)

pom.xml

@@ -42,6 +42,11 @@
   </scm>
 
   <dependencies>
+    <dependency>
+      <groupId>io.jenkins.plugins</groupId>
+      <artifactId>commons-lang3-api</artifactId>
+    </dependency>
+
     <dependency>
       <groupId>io.jenkins.plugins</groupId>
       <artifactId>plugin-util-api</artifactId>