[7.x](backport #27509) Add a header round tripper option to httpcommon

[mergify]

This is an automatic backport of pull request #27509 done by Mergify.
Cherry-pick of 8a5dac6 has failed:

On branch mergify/bp/7.x/pr-27509
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 8a5dac67f9.
  (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:   dev-tools/cmd/dashboards/export_dashboards.go
	modified:   filebeat/beater/filebeat.go
	modified:   heartbeat/monitors/active/http/http.go
	modified:   heartbeat/monitors/active/http/http_test.go
	modified:   libbeat/cmd/instance/beat.go
	modified:   libbeat/common/transport/httpcommon/httpcommon.go
	modified:   libbeat/dashboards/dashboards.go
	modified:   libbeat/dashboards/kibana_loader.go
	modified:   libbeat/esleg/eslegclient/connection.go
	modified:   libbeat/kibana/client.go
	modified:   libbeat/kibana/client_test.go
	modified:   libbeat/monitoring/report/elasticsearch/elasticsearch.go
	modified:   libbeat/outputs/elasticsearch/client.go
	modified:   libbeat/outputs/elasticsearch/elasticsearch.go
	modified:   metricbeat/helper/http.go
	modified:   metricbeat/helper/http_test.go
	modified:   x-pack/elastic-agent/pkg/agent/cmd/container.go
	modified:   x-pack/filebeat/input/httpjson/input.go
	modified:   x-pack/filebeat/input/httpjson/requester.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   heartbeat/monitors/active/http/task.go
	both modified:   heartbeat/monitors/active/http/task_test.go
	both modified:   libbeat/cmd/export/dashboard.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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• Start Time: 2021-09-08T18:33:55.272+0000

• Duration: 150 min 19 sec

• Commit: 9a30039

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Total	58578

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Passed	53310
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Total	58578