Add aggregation support

[obukhow]

Add aggregation support for queries

An aggregation summarizes your data as metrics, statistics, or other analytics. https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations.html

Description

Started by implementing two most common used aggregations – terms bucket and stats metric aggregation. Other aggregations type could be implemented by extending AggregationInterface

• [ ] Bug fix
• [x] Feature addition
• [ ] Code refactoring
• [ ] Documentation update
• [ ] Other (please explain):

Related Issue

How Has This Been Tested?

• [x] Unit tests
• [ ] Integration tests
• [x] Manual testing

Checklist

• [x] My code follows the style guidelines of this project.
• [x] I have performed a self-review of my code.
• [ ] I have made corresponding changes to the documentation.
• [x] I have added tests that ensure my fix is effective or that my feature works.
• [x] New and existing unit tests pass locally with my changes.
• [ ] Any dependent changes have been merged and published in downstream modules.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Files with missing lines	Coverage Δ
effdsl.go	69.30% <100.00%> (+3.00%)	:arrow_up:
search_aggregation.go	100.00% <100.00%> (ø)

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