Add reference listener to dynamically add schema mixin annotations

[gashcrumb]

A lot of objects that we turn into schemas are going to probably need tweaking here and there to avoid circular references or add metadata and what-not. We should add a reference listener to schema lookup so that mixin annotations can be easily added without adding loads of dependencies on hawtio-json-schema-mbean to handle these mixins

[gashcrumb]

Was implemented but reference lists aren't supported in blueprint-web yet, so we'll build annotation mixins into hawtio-json-schema-mbean for now. Could also investigate adding a service so that bundles could register mixin annotations directly, however I suspect that proxy usage in karaf might render this solution a no-go.