[JENKINS-28150] Fix optional dependency loading

The extensions here that are using an optional dependency's class must
be mark as optional. If not, we can see `ClassNotFoundException` when
Jenkins load this plugin.

src/main/java/com/synopsys/arc/jenkinsci/plugins/run_condition_extras/adapters/build_timeout/ConditionalTimeout.java

@@ -68,7 +68,7 @@ public Descriptor<ConditionalTimeout> getDescriptor() {
         return DESCRIPTOR;
     }
 
-    @Extension
+    @Extension(optional = true)
     public static final DescriptorImpl DESCRIPTOR = new DescriptorImpl();
 
     public static class DescriptorImpl extends Descriptor<ConditionalTimeout> {

src/main/java/com/synopsys/arc/jenkinsci/plugins/run_condition_extras/adapters/build_timeout/RunConditionTimeoutStrategy.java

@@ -89,7 +89,7 @@ public Descriptor<BuildTimeOutStrategy> getDescriptor() {
         return DESCRIPTOR;
     }
 
-    @Extension
+    @Extension(optional = true)
     public static final DescriptorImpl DESCRIPTOR = new DescriptorImpl();
 
     public static class DescriptorImpl extends BuildTimeOutStrategyDescriptor {

src/main/java/com/synopsys/arc/jenkinsci/plugins/run_condition_extras/adapters/mail_ext/RunConditionEmailTrigger.java

@@ -76,7 +76,7 @@ public RunCondition getCondition() {
         return condition;
     }
 
-    @Extension
+    @Extension(optional = true)
     public static final DescriptorImpl DESCRIPTOR = new DescriptorImpl();
 
     @Override