implement scan for composite aggregation

[pgtuk]

What do these changes do?

Added convenient scan for composite aggregations as specified in es docs.

Are there changes in behavior for the user?

None.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Merging #225 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master    #225      +/-   ##
=========================================
- Coverage    0.23%   0.19%   -0.04%     
=========================================
  Files           6       6              
  Lines         434     520      +86     
  Branches       76      89      +13     
=========================================
  Hits            1       1              
- Misses        433     519      +86     

Impacted Files	Coverage Δ
aioelasticsearch/helpers.py	0.00% <0.00%> (ø)

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