[8.19](backport #44892) x-pack/filebeat/input/gcppubsub: add support for proxy configuration

[mergify]

Proposed commit message

x-pack/filebeat/input/gcppubsub: add support for proxy configuration

This is a minimal addition for addition of proxy configuration. To
acheive this, the full httpcommon.HTTPTransportSettings struct is not
used since it brings defaults that may change behaviour for users.
Instead it uses a subset of the struct using otherwise zero values for
the rest when the proxy-aware client is constructed.

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.

Disruptive User Impact

Author's Checklist

• [ ]

How to test this PR locally

Related issues

Use cases

Screenshots

Logs

---

This is an automatic backport of pull request #44892 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 875af7d has failed:

On branch mergify/bp/8.19/pr-44892
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit 875af7d9e.
  (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/input/gcppubsub/config.go
	modified:   x-pack/filebeat/input/gcppubsub/input.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   docs/reference/filebeat/filebeat-input-gcp-pubsub.md

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

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)