[7.x](backport #25710) Add multiline support to awss3 input

[mergify]

This is an automatic backport of pull request #25710 done by Mergify.
Cherry-pick of 5f242e3 has failed:

On branch mergify/bp/7.x/pr-25710
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 5f242e3c3d.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   x-pack/filebeat/docs/inputs/input-aws-s3.asciidoc
	modified:   x-pack/filebeat/input/awss3/collector_test.go
	modified:   x-pack/filebeat/input/awss3/config.go
	new file:   x-pack/filebeat/input/awss3/ftest/sample2.txt
	modified:   x-pack/filebeat/input/awss3/s3_integration_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/filebeat/input/awss3/collector.go

To fix up this pull request, you can check it out locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #25740 updated

• Start Time: 2021-05-18T15:56:11.020+0000

• Duration: 111 min 49 sec

• Commit: e4b5c1d

Test stats 🧪

Test	Results
Failed	0
Passed	7002
Skipped	1193
Total	8195

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	7002
Skipped	1193
Total	8195

[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-25710 upstream/mergify/bp/7.x/pr-25710
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-25710

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[andrewkroh]

@Mergifyio rebase

[mergify]

Command rebase: success

Branch has been successfully rebased