Add required attribute to <interceptor> tag in beans.xml

Currently the deployment will fail if an interceptor is not present, which means that it is not conventient to use interceptors from optional modules, as the end user will need to open up the jar file and edit the beans.xml file manually.

Adding a required="true|false" attribute to the <interceptor> element in beans.xml would allow an archive to specify that the interceptor is not essential, and if it is not found it should simply be ignored.

For example, currently Seam Security has a hard dependency on Seam Persistence because it uses the Transaction interceptor, and the deployment will fail if Seam Persistence is not present.