Add support for file preview

[huyaboo]

Description

This PR adds two new routes:

• _preview
• _cat_indices

And modifies two routes:

• _import_file
• _import_text

in order to support a file-preview like experience. Chiefly, the _preview route is used for parsing the file and returning the best guess index mapping that the documents will have. This PR does not cover the case where the existing index mapping and predicted index mapping differ. This is up to UI

Issues Resolved

Screenshot

Testing the changes

Changelog

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

[github-actions]

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[github-actions]

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[codecov]

Codecov Report

Attention: Patch coverage is 72.28916% with 23 lines in your changes missing coverage. Please review.

Please upload report for BASE (feature/data-importer@74dc8cc). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...a_importer/public/components/data_importer_app.tsx	11.11%	8 Missing ⚠️
src/plugins/data_importer/public/lib/preview.ts	0.00%	5 Missing ⚠️
...plugins/data_importer/server/parsers/csv_parser.ts	76.47%	3 Missing and 1 partial ⚠️
src/plugins/data_importer/server/utils/util.ts	89.65%	1 Missing and 2 partials ⚠️
...lugins/data_importer/server/parsers/json_parser.ts	83.33%	2 Missing ⚠️
...gins/data_importer/server/parsers/ndjson_parser.ts	90.90%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##             feature/data-importer    #9354   +/-   ##
========================================================
  Coverage                         ?   61.75%           
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  Files                            ?     3832           
  Lines                            ?    92144           
  Branches                         ?    14596           
========================================================
  Hits                             ?    56902           
  Misses                           ?    31572           
  Partials                         ?     3670           

Flag	Coverage Δ
Linux_1	28.99% <ø> (?)
Linux_2	56.46% <ø> (?)
Linux_4	28.91% <ø> (?)
Windows_1	29.02% <ø> (?)
Windows_2	56.41% <ø> (?)
Windows_3	39.35% <72.28%> (?)
Windows_4	28.91% <ø> (?)

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