refactored aggs implementation

src/Aggregation/AbstractAggregation.php

src/Aggregation/GlobalAggregation.php

src/Aggregation/ReverseNestedAggregation.php

src/Search.php

src/SearchEndpoint/AbstractSearchEndpoint.php

src/SearchEndpoint/AggregationsEndpoint.php

src/SearchEndpoint/HighlightEndpoint.php

src/SearchEndpoint/SearchEndpointInterface.php

tests/Aggregation/FilterAggregationTest.php

tests/Aggregation/FiltersAggregationTest.php

tests/Aggregation/GeoBoundsAggregationTest.php

tests/Aggregation/GeoDistanceAggregationTest.php

tests/Aggregation/GlobalAggregationTest.php

tests/Aggregation/Ipv4RangeAggregationTest.php

tests/Aggregation/NestedAggregationTest.php

tests/Aggregation/PercentileRanksAggregationTest.php

tests/Aggregation/RangeAggregationTest.php

tests/Aggregation/ReverseNestedAggregationTest.php

tests/Aggregation/StatsAggregationTest.php

tests/Aggregation/TermsAggregationTest.php