moved all metric and bucketing aggs to its kind of namespace

src/Aggregation/AvgAggregation.php

src/Aggregation/Bucketing/ChildrenAggregation.php

src/Aggregation/Bucketing/DateHistogramAggregation.php

src/Aggregation/Bucketing/DateRangeAggregation.php

src/Aggregation/Bucketing/FilterAggregation.php

src/Aggregation/Bucketing/FiltersAggregation.php

src/Aggregation/Bucketing/GeoDistanceAggregation.php

src/Aggregation/Bucketing/GeoHashGridAggregation.php

src/Aggregation/Bucketing/GlobalAggregation.php

src/Aggregation/Bucketing/HistogramAggregation.php

src/Aggregation/Bucketing/Ipv4RangeAggregation.php

src/Aggregation/Bucketing/MissingAggregation.php

src/Aggregation/Bucketing/NestedAggregation.php

src/Aggregation/Bucketing/RangeAggregation.php

src/Aggregation/Bucketing/ReverseNestedAggregation.php

src/Aggregation/Bucketing/SamplerAggregation.php

src/Aggregation/Bucketing/SignificantTermsAggregation.php

src/Aggregation/Bucketing/TermsAggregation.php

src/Aggregation/CardinalityAggregation.php

src/Aggregation/ChildrenAggregation.php