Add composite aggregation

[rdettai-sk]

See https://www.elastic.co/docs/reference/aggregations/search-aggregations-bucket-composite-aggregation

Composite aggregations are usefull to export pre-aggregated data at medium cardinailities (10k to millions).

[PSeitz-dd]

@rdettai-sk I refactored aggregations in this PR: #2715

[rdettai-sk]

Test Name	Memory	Avg	Median	Range
terms_few	27.9 KB	1.9207ms	1.8848ms	[1.8192ms .. 3.1971ms]
terms_many	6.9 MB	5.7860ms	5.6407ms	[5.4616ms .. 8.0505ms]
terms_many_top_1000	6.9 MB	7.3430ms	7.2118ms	[6.9790ms .. 9.6555ms]
terms_many_with_avg_sub_agg	27.8 MB	42.5193ms	41.1872ms	[38.9443ms .. 50.6991ms]
composite_term_many_page_1000	436.8 KB	16.4085ms	16.2403ms	[15.9183ms .. 19.1130ms]
composite_term_many_page_1000_with_avg_sub_agg	2.0 MB	18.2566ms	18.0860ms	[17.0605ms .. 20.9226ms]
composite_term_few	28.7 KB	12.2631ms	12.0812ms	[11.7445ms .. 15.8399ms]

[rdettai-sk]

Impact of the refacto:

Test Name	Memory	Avg	Median	Range
composite_term_many_page_1000	435.8 KB (-0.15%)	15.7835ms (-6.12%)	15.6270ms (-7.18%)	[15.2078ms .. 19.8406ms]
composite_term_many_page_1000_with_avg_sub_agg	2.0 MB (-0.08%)	16.9791ms (-7.76%)	17.0189ms (-8.24%)	[15.9805ms .. 17.7368ms]
composite_term_few	28.3 KB (-2.28%)	11.7407ms (-2.95%)	11.7378ms (-2.83%)	[11.5437ms .. 11.9527ms]

[rdettai-sk]

@PSeitz this PR is finally pretty much ready. Histogram benchmarks are a bit disappointing, I'll try to figure out why.

Benchmark	Memory	Avg	Median	Range
composite_term_many_page_1000	435.9 KB	17.1144ms	17.0872ms	[16.5733ms .. 17.5875ms]
composite_term_many_page_1000_with_avg_sub_agg	2.0 MB	18.4522ms	18.4074ms	[17.2280ms .. 19.0033ms]
composite_term_few	28.2 KB	12.1930ms	12.0968ms	[11.6045ms .. 15.3977ms]
composite_histogram	304.0 KB	57.7002ms	57.4315ms	[56.1724ms .. 59.2285ms]
composite_histogram_calendar	156.2 KB	24.1235ms	24.0230ms	[23.3956ms .. 25.9654ms]