Cherry-pick #18370 to 7.8: [Filebeat] Unescape characters in s3 file names

[kaiyan-sheng]

Cherry-pick of PR #18370 to 7.8 branch. Original message:

What does this PR do?

This PR unescape 3-byte encoded substring to regular string. For example: "%3D" to "=".

Why is it important?

When user uses folders in S3 bucket to organize log files, SQS notification actually encode the log file path like below:

{"Records":[{"eventVersion":"2.1","eventSource":"aws:s3","awsRegion":"us-east-1","eventTime":"2020-05-07T21:02:38.676Z","eventName":"ObjectCreated:Put","userIdentity":{"principalId":"AWS:AIDAWHL7AXDB2IME26NB3"},"requestParameters":{"sourceIPAddress":"174.29.210.150"},"responseElements":{"x-amz-request-id":"B86806D8E85F4A5F","x-amz-id-2":"yw1VtzJIcckqIpEjmp/oeesJj3yplnGR48PDCX0/nS8Qj5VXWGdzCrjSzAv0cRio/oAXihdDwscZE7pm32CHQH8gbjM60Qu6"},"s3":{"s3SchemaVersion":"1.0","configurationId":"ObjectCreated","bucket":{"name":"test-fb-ks","ownerIdentity":{"principalId":"A2EOMKP5A4DS45"},"arn":"arn:aws:s3:::test-fb-ks"},"object":{"key":"year%3D2020/month%3D05/test1.txt","size":6,"eTag":"3e7705498e8be60520841409ebc69bc1","sequencer":"005EB47773C10D8A81"}}}]}

File in S3 is year=2020/month=05/test1.txt but in SQS is converted to year%3D2020/month%3D05/test1.txt.

This conversion needs to be undo when Filebeat tries to read the S3 file pointed by SQS message. Otherwise, this file will not be found.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Related issues

https://discuss.elastic.co/t/filebeat-s3-issue/230441

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

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Build stats

• Build Cause: [Pull request Cherry-pick #18370 to 7.8: [Filebeat] Unescape characters in s3 file names #18412 updated]

• Start Time: 2020-05-11T14:42:11.306+0000

• Duration: 46 min 48 sec (2748393)

• Commit: b34d08e

Test stats 🧪

Test	Results
Failed	0
Passed	1182
Skipped	128
Total	1310