Aggregations: Added getProperty method to Aggregations

This allows arbitrary properties to be retrieved from an aggregation tree. The property is specified using the same syntax as the
order parameter in the terms aggregation. If a property path contians a multi-bucket aggregation the property values from each bucket will be returned in an array.
This can't be tested or used as yet because of the aggregation name problem you mentioned below. When we have resolved #8434 we can test and use this functionality