[7.x](backport #27168) [Filebeat] Add HMAC template function for httpjson input

[mergify]

This is an automatic backport of pull request #27168 done by Mergify.

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[elasticmachine]

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• Start Time: 2021-08-04T09:07:37.136+0000

• Duration: 91 min 14 sec

• Commit: 6265dc5

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Test	Results
Failed	0
Passed	7475
Skipped	1201
Total	8676

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Test stats 🧪

Test	Results
Failed	0
Passed	7475
Skipped	1201
Total	8676

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/7.x/pr-27168 upstream/mergify/bp/7.x/pr-27168
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-27168