Use java-style annotations for metadata

Drools Metadata, allowed in rules and bean declarations, are conceptually similar to java annotations, but the actual structure is different: Drools is free-form, while Java uses (optional) key - element pairs:

Drools : @attr( <free text here> )
Java : @attr( key1 = value1, key2 ... )

I suggest the use of Java-like annotations in place of free-form meta-attributes.

Compatibility with official metadata (e.g. event definition) will be guaranteed.
Could break compatibility if a user defined and used their own metadata in the code, as some constructs like @author(john doe) will no longer be accepted, even if @author("john doe"), @author(john, doe) or @author(name="john doe") will.