Allow `Attribute` to query an MBean operation

Allow `Attribute` to query an MBean operation

MBeans provide two entities to the outside world:

* Attributes
* Operations

The former is a simple key/value lookup, while the latter
triggers a method call and yields the result.

Operations have signatures, they can be fed an array of
parameters. For now, calls are triggered assuming
there are no parameters. Later a separate keyword
`Operation` can be introduced, taking a list of
`Parameter` directives specifying type and values
to feed to the MBean operation.