[7.x](backport #26514) Enroll proxy settings

[mergify]

This is an automatic backport of pull request #26514 done by Mergify.
Cherry-pick of 26325b0 has failed:

On branch mergify/bp/7.x/pr-26514
Your branch is up to date with 'origin/7.x'.

You are currently cherry-picking commit 26325b01d.
  (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:   x-pack/elastic-agent/CHANGELOG.next.asciidoc
	modified:   x-pack/elastic-agent/pkg/agent/cmd/enroll.go
	modified:   x-pack/elastic-agent/pkg/agent/configuration/fleet_server.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/elastic-agent/pkg/agent/cmd/enroll_cmd.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 #26610 updated

• Start Time: 2021-07-06T11:50:03.136+0000

• Duration: 75 min 13 sec

• Commit: 69403c3

Test stats 🧪

Test	Results
Failed	0
Passed	6924
Skipped	24
Total	6948

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	6924
Skipped	24
Total	6948

[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-26514 upstream/mergify/bp/7.x/pr-26514
git merge upstream/7.x
git push upstream mergify/bp/7.x/pr-26514