composite aggregation filter (#15)

[benwalch]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Fixed tickets	#15, partially

This PR provides a composite filter / aggregation.

This filter works exactly like the aggregation filter, except the fields on which should be aggregated / filtered, which can be a combination of multiple fields.

Example:

->addFilterDefinition([
    'name'          => 'my_composite_agg'
    'type'          => 'composite_aggregation',
    'configuration' => [
        'fields'              => ['field1', 'field2, 'fieldX', ...],
        'separator'       => '--' // default: '__',
        ...
    ]
])

The composite fields will be aggregated as follows

"filter": [
    {
        "name": "my_composite_agg",
        ...
        "values": [
            {
                "value": "field1valueA--field2valueB--fieldXvalueC",
                ...
            },
            {
                "value": "field1valueK--field2valueB--fieldXvalueY",
                ...
            },
            ...