[7.x](backport #28385) [filebeat] Add base64Decode template functions to httpjson

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This is an automatic backport of pull request #28385 done by Mergify.

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• Start Time: 2021-10-14T11:54:13.428+0000

• Duration: 120 min 40 sec

• Commit: 4593c5b

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Test	Results
Failed	0
Passed	8283
Skipped	1201
Total	9484

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