Update PageProvider interface and service so as to include Aggregates support

• a method for getting the aggregates. Aggregates should be typed.
• a contribution for specifying aggregates that should be fetched, unless this is done in NXQL (to be specified)
• an implementation of the construction of the query for elastic search, in the ElasticSearchPageProvider.

Unsupported on other Page providers.

About contribution for specifying aggregates
This involves to define the descriptors to be added to the page provider definition in order to contribute all the aggregates that will have to be returned along with the query result.

About an aggregate:

• it has a unique id within the scope of a content view
• it is on a particular document property.
• it has a type

Aggregate types can be found in ES doc. We'll first foucs on two types:

• cardinality (for string document properties)
• histogram (for date/numeric document properties)

Depending on the type of the aggregate, additional information options to be added in the page provider definition. For instance, the cardinality have a precision_threshold option and histogram have interval option. This kind of option will be given dynamically from the higher layer but we might want to set default values.